| Message ID | b064d09d94fb2a15ad72427962df400e37788d0c.1677026757.git.boris@bur.io (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | btrfs: dio partial write corruption fix | expand |
On Wed, Feb 22, 2023 at 1:04 AM Boris Burkov <boris@bur.io> wrote: > > If an application is doing direct io to a btrfs file and experiences a > page fault reading from the write buffer, iomap will issue a partial > bio, and allow the fs to keep going. However, there was a subtle bug in > this codepath in the btrfs dio iomap implementation that led to the > partial write ending up as a gap in the file's extents and to be read > back as zeros. > > The sequence of events in a partial write, lightly summarized and > trimmed down for brevity is as follows: > > ====WRITING TASK==== > btrfs_direct_write > __iomap_dio_write > iomap_iter > btrfs_dio_iomap_begin # create full ordered extent > iomap_dio_bio_iter > bio_iov_iter_get_pages # page fault; partial read > submit_bio # partial bio > iomap_iter > btrfs_dio_iomap_end > btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; > # submit to finish_ordered_fn wq > fault_in_iov_iter_readable # btrfs_direct_write detects partial write > __iomap_dio_write > iomap_iter > btrfs_dio_iomap_begin # create second partial ordered extent > iomap_dio_bio_iter > bio_iov_iter_get_pages # read all of remainder > submit_bio # partial bio with all of remainder > iomap_iter > btrfs_dio_iomap_end # nothing exciting to do with ordered io > > ====DIO ENDIO==== > ==FIRST PARTIAL BIO== > btrfs_dio_end_io > btrfs_mark_ordered_io_finished # bytes_left > 0 > # don't submit to finish_ordered_fn wq > ==SECOND PARTIAL BIO== > btrfs_dio_end_io > btrfs_mark_ordered_io_finished # bytes_left == 0 > # submit to finish_ordered_fn wq > > ====BTRFS FINISH ORDERED WQ==== > ==FIRST PARTIAL BIO== > btrfs_finish_ordered_io # called by dio_iomap_end_io, sees > # BTRFS_ORDERED_IOERR, just drops the > # ordered_extent > ==SECOND PARTIAL BIO== > btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file > # extents, csums, etc... Thanks, this makes it a lot more clear for anyone trying to understand the problem. > > The essence of the problem is that while btrfs_direct_write and iomap > properly interact to submit all the correct bios, there is insufficient > logic in the btrfs dio functions (btrfs_dio_iomap_begin, > btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to > ensure that every bio is at least a part of a completed ordered_extent. > And it is completing an ordered_extent that results in crucial > functionality like writing out a file extent for the range. " writing out a file extent for the range", is kind of ambiguous, more like inserting a file extent item for the range in the subvolume/fs tree. > > More specifically, btrfs_dio_end_io treats the ordered extent as > unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. > Thus, the finish io work doesn't result in file extents, csums, etc... > In the aftermath, such a file behaves as though it has a hole in it, > instead of the purportedly written data. > > We considered a few options for fixing the bug (apologies for any > incorrect summary of a proposal which I didn't implement and fully > understand): > 1. treat the partial bio as if we had truncated the file, which would > result in properly finishing it. > 2. split the ordered extent when submitting a partial bio. > 3. cache the ordered extent across calls to __iomap_dio_rw in > iter->private, so that we could reuse it and correctly apply several > bios to it. > > I had trouble with 1, and it felt the most like a hack, so I tried 2 > and 3. Since 3 has the benefit of also not creating an extra file > extent, and avoids an ordered extent lookup during bio submission, it > felt like the best option. > > A quick summary of the changes necessary to implement this cached > ordered_extent behavior: > - btrfs_direct_write keeps track of an ordered_extent for the duration > of a call, possible across several __iomap_dio_rws. > - zero the btrfs_dio_data before using it, since its fields constitute > state now. > - btrfs_dio_write uses dio_data to pass this ordered extent into and out > of __iomap_dio_rw. > - when the write is done, put the ordered_extent. > - if the short write happens to be length 0, then we _don't_ get an > extra bio, so we do need to cancel the ordered_extent like we used > to (and ditch the cached ordered extent) > - in btrfs_dio_iomap_begin, if the cached ordered extent is present, > skip all the work of creating it, just look up the extent mapping and > jump to setting up the iomap. (This part could likely be more > elegant..) > > Thanks to Josef, Christoph, and Filipe with their help figuring out the > bug and the fix. > > Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 > Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ > Link: https://pastebin.com/3SDaH8C6 > Signed-off-by: Boris Burkov <boris@bur.io> > --- > fs/btrfs/btrfs_inode.h | 1 + > fs/btrfs/file.c | 11 ++++++- > fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- > 3 files changed, 67 insertions(+), 20 deletions(-) > > diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h > index 49a92aa65de1..87020aa58121 100644 > --- a/fs/btrfs/btrfs_inode.h > +++ b/fs/btrfs/btrfs_inode.h > @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, > size_t done_before); > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > + struct btrfs_ordered_extent **ordered_extent, > size_t done_before); > > extern const struct dentry_operations btrfs_dentry_operations; > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > index 5cc5a1faaef5..ec5c5355906b 100644 > --- a/fs/btrfs/file.c > +++ b/fs/btrfs/file.c > @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > ssize_t err; > unsigned int ilock_flags = 0; > struct iomap_dio *dio; > + struct btrfs_ordered_extent *ordered_extent = NULL; > > if (iocb->ki_flags & IOCB_NOWAIT) > ilock_flags |= BTRFS_ILOCK_TRY; > @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > * got -EFAULT, faulting in the pages before the retry. > */ > from->nofault = true; > - dio = btrfs_dio_write(iocb, from, written); > + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); > from->nofault = false; > > /* > @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > goto relock; > } > } > + /* > + * We can't loop back to btrfs_dio_write, so we can drop the cached > + * ordered extent. Typically btrfs_dio_iomap_end will run and put the > + * ordered_extent, but this is needed to clean up in case of an error > + * path breaking out of iomap_iter before the final iomap_end call. > + */ > + if (ordered_extent) > + btrfs_put_ordered_extent(ordered_extent); > > /* > * If 'err' is -ENOTBLK or we have not written all data, then it means > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > index 44e9acc77a74..f1a59c5f3140 100644 > --- a/fs/btrfs/inode.c > +++ b/fs/btrfs/inode.c > @@ -81,6 +81,7 @@ struct btrfs_dio_data { > struct extent_changeset *data_reserved; > bool data_space_reserved; > bool nocow_done; > + struct btrfs_ordered_extent *ordered; > }; > > struct btrfs_dio_private { > @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, > } > > static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > + struct btrfs_dio_data *dio_data, > const u64 start, > const u64 len, > const u64 orig_start, > @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > const int type) > { > struct extent_map *em = NULL; > - int ret; > + struct btrfs_ordered_extent *ordered; > > if (type != BTRFS_ORDERED_NOCOW) { > em = create_io_em(inode, start, len, orig_start, block_start, > @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > if (IS_ERR(em)) > goto out; > } > - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, > - block_len, 0, > - (1 << type) | > - (1 << BTRFS_ORDERED_DIRECT), > - BTRFS_COMPRESS_NONE); > - if (ret) { > + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, > + block_start, block_len, 0, > + (1 << type) | > + (1 << BTRFS_ORDERED_DIRECT), > + BTRFS_COMPRESS_NONE); > + if (IS_ERR(ordered)) { > if (em) { > free_extent_map(em); > btrfs_drop_extent_map_range(inode, start, > start + len - 1, false); > } > - em = ERR_PTR(ret); > + em = ERR_PTR(PTR_ERR(ordered)); > + } else { > + ASSERT(!dio_data->ordered); > + dio_data->ordered = ordered; > } > out: > > @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > } > > static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > + struct btrfs_dio_data *dio_data, > u64 start, u64 len) > { > struct btrfs_root *root = inode->root; > @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > if (ret) > return ERR_PTR(ret); > > - em = btrfs_create_dio_extent(inode, start, ins.offset, start, > + em = btrfs_create_dio_extent(inode, dio_data, > + start, ins.offset, start, > ins.objectid, ins.offset, ins.offset, > ins.offset, BTRFS_ORDERED_REGULAR); > btrfs_dec_block_group_reservations(fs_info, ins.objectid); > @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > } > space_reserved = true; > > - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, > + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, > + start, len, > orig_start, block_start, > len, orig_block_len, > ram_bytes, type); > @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > goto out; > space_reserved = true; > > - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); > + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); > if (IS_ERR(em)) { > ret = PTR_ERR(em); > goto out; > @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > } > } > > + if (dio_data->ordered) { > + ASSERT(write); > + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, > + dio_data->ordered->file_offset, > + dio_data->ordered->bytes_left); > + if (IS_ERR(em)) { > + ret = PTR_ERR(em); > + goto err; > + } > + goto map_iomap; > + } > memset(dio_data, 0, sizeof(*dio_data)); > > /* > @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > else > free_extent_state(cached_state); > > +map_iomap: > /* > * Translate extent map information to iomap. > * We trim the extents (and move the addr) even though iomap code does > @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, > if (submitted < length) { > pos += submitted; > length -= submitted; > - if (write) > - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, > - pos, length, false); > - else > + if (write) { > + if (submitted == 0) { > + btrfs_mark_ordered_io_finished(BTRFS_I(inode), > + NULL, pos, > + length, false); > + btrfs_put_ordered_extent(dio_data->ordered); > + dio_data->ordered = NULL; > + } > + } else { > unlock_extent(&BTRFS_I(inode)->io_tree, pos, > pos + length - 1, NULL); > + } > ret = -ENOTBLK; > + } else { > + /* On the last bio, release our cached ordered_extent */ > + if (write) { > + btrfs_put_ordered_extent(dio_data->ordered); > + dio_data->ordered = NULL; > + } > } > > if (write) > @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > { > - struct btrfs_dio_data data; > + struct btrfs_dio_data data = { }; Btw, everywhere else we use the { 0 } style, so we should, ideally, be consistent and use it here too. Those are just minor things, David can fix them up when he picks the patch if he wants to, so: Reviewed-by: Filipe Manana <fdmanana@suse.com> > > return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > IOMAP_DIO_PARTIAL, &data, done_before); > } > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > + struct btrfs_ordered_extent **ordered_extent, > size_t done_before) > { > - struct btrfs_dio_data data; > + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; > + struct iomap_dio *dio; > > - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > - IOMAP_DIO_PARTIAL, &data, done_before); > + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > + IOMAP_DIO_PARTIAL, &dio_data, done_before); > + if (!IS_ERR_OR_NULL(dio)) > + *ordered_extent = dio_data.ordered; > + return dio; > } > > static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > -- > 2.38.1 >
On Wed, Feb 22, 2023 at 11:51 AM Filipe Manana <fdmanana@kernel.org> wrote: > > On Wed, Feb 22, 2023 at 1:04 AM Boris Burkov <boris@bur.io> wrote: > > > > If an application is doing direct io to a btrfs file and experiences a > > page fault reading from the write buffer, iomap will issue a partial > > bio, and allow the fs to keep going. However, there was a subtle bug in > > this codepath in the btrfs dio iomap implementation that led to the > > partial write ending up as a gap in the file's extents and to be read > > back as zeros. > > > > The sequence of events in a partial write, lightly summarized and > > trimmed down for brevity is as follows: > > > > ====WRITING TASK==== > > btrfs_direct_write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create full ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # page fault; partial read > > submit_bio # partial bio > > iomap_iter > > btrfs_dio_iomap_end > > btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; > > # submit to finish_ordered_fn wq > > fault_in_iov_iter_readable # btrfs_direct_write detects partial write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create second partial ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # read all of remainder > > submit_bio # partial bio with all of remainder > > iomap_iter > > btrfs_dio_iomap_end # nothing exciting to do with ordered io > > > > ====DIO ENDIO==== > > ==FIRST PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left > 0 > > # don't submit to finish_ordered_fn wq > > ==SECOND PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left == 0 > > # submit to finish_ordered_fn wq > > > > ====BTRFS FINISH ORDERED WQ==== > > ==FIRST PARTIAL BIO== > > btrfs_finish_ordered_io # called by dio_iomap_end_io, sees > > # BTRFS_ORDERED_IOERR, just drops the > > # ordered_extent > > ==SECOND PARTIAL BIO== > > btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file > > # extents, csums, etc... > > Thanks, this makes it a lot more clear for anyone trying to understand > the problem. > > > > > The essence of the problem is that while btrfs_direct_write and iomap > > properly interact to submit all the correct bios, there is insufficient > > logic in the btrfs dio functions (btrfs_dio_iomap_begin, > > btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to > > ensure that every bio is at least a part of a completed ordered_extent. > > And it is completing an ordered_extent that results in crucial > > functionality like writing out a file extent for the range. > > " writing out a file extent for the range", is kind of ambiguous, more > like inserting a file extent item > for the range in the subvolume/fs tree. > > > > > More specifically, btrfs_dio_end_io treats the ordered extent as > > unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. > > Thus, the finish io work doesn't result in file extents, csums, etc... > > In the aftermath, such a file behaves as though it has a hole in it, > > instead of the purportedly written data. > > > > We considered a few options for fixing the bug (apologies for any > > incorrect summary of a proposal which I didn't implement and fully > > understand): > > 1. treat the partial bio as if we had truncated the file, which would > > result in properly finishing it. > > 2. split the ordered extent when submitting a partial bio. > > 3. cache the ordered extent across calls to __iomap_dio_rw in > > iter->private, so that we could reuse it and correctly apply several > > bios to it. > > > > I had trouble with 1, and it felt the most like a hack, so I tried 2 > > and 3. Since 3 has the benefit of also not creating an extra file > > extent, and avoids an ordered extent lookup during bio submission, it > > felt like the best option. > > > > A quick summary of the changes necessary to implement this cached > > ordered_extent behavior: > > - btrfs_direct_write keeps track of an ordered_extent for the duration > > of a call, possible across several __iomap_dio_rws. > > - zero the btrfs_dio_data before using it, since its fields constitute > > state now. > > - btrfs_dio_write uses dio_data to pass this ordered extent into and out > > of __iomap_dio_rw. > > - when the write is done, put the ordered_extent. > > - if the short write happens to be length 0, then we _don't_ get an > > extra bio, so we do need to cancel the ordered_extent like we used > > to (and ditch the cached ordered extent) > > - in btrfs_dio_iomap_begin, if the cached ordered extent is present, > > skip all the work of creating it, just look up the extent mapping and > > jump to setting up the iomap. (This part could likely be more > > elegant..) > > > > Thanks to Josef, Christoph, and Filipe with their help figuring out the > > bug and the fix. > > > > Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 > > Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ > > Link: https://pastebin.com/3SDaH8C6 > > Signed-off-by: Boris Burkov <boris@bur.io> > > --- > > fs/btrfs/btrfs_inode.h | 1 + > > fs/btrfs/file.c | 11 ++++++- > > fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- > > 3 files changed, 67 insertions(+), 20 deletions(-) > > > > diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h > > index 49a92aa65de1..87020aa58121 100644 > > --- a/fs/btrfs/btrfs_inode.h > > +++ b/fs/btrfs/btrfs_inode.h > > @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, > > size_t done_before); > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before); > > > > extern const struct dentry_operations btrfs_dentry_operations; > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > > index 5cc5a1faaef5..ec5c5355906b 100644 > > --- a/fs/btrfs/file.c > > +++ b/fs/btrfs/file.c > > @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > ssize_t err; > > unsigned int ilock_flags = 0; > > struct iomap_dio *dio; > > + struct btrfs_ordered_extent *ordered_extent = NULL; > > > > if (iocb->ki_flags & IOCB_NOWAIT) > > ilock_flags |= BTRFS_ILOCK_TRY; > > @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > * got -EFAULT, faulting in the pages before the retry. > > */ > > from->nofault = true; > > - dio = btrfs_dio_write(iocb, from, written); > > + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); > > from->nofault = false; > > > > /* > > @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > goto relock; > > } > > } > > + /* > > + * We can't loop back to btrfs_dio_write, so we can drop the cached > > + * ordered extent. Typically btrfs_dio_iomap_end will run and put the > > + * ordered_extent, but this is needed to clean up in case of an error > > + * path breaking out of iomap_iter before the final iomap_end call. > > + */ > > + if (ordered_extent) > > + btrfs_put_ordered_extent(ordered_extent); > > > > /* > > * If 'err' is -ENOTBLK or we have not written all data, then it means > > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > > index 44e9acc77a74..f1a59c5f3140 100644 > > --- a/fs/btrfs/inode.c > > +++ b/fs/btrfs/inode.c > > @@ -81,6 +81,7 @@ struct btrfs_dio_data { > > struct extent_changeset *data_reserved; > > bool data_space_reserved; > > bool nocow_done; > > + struct btrfs_ordered_extent *ordered; > > }; > > > > struct btrfs_dio_private { > > @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > const u64 start, > > const u64 len, > > const u64 orig_start, > > @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > const int type) > > { > > struct extent_map *em = NULL; > > - int ret; > > + struct btrfs_ordered_extent *ordered; > > > > if (type != BTRFS_ORDERED_NOCOW) { > > em = create_io_em(inode, start, len, orig_start, block_start, > > @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > if (IS_ERR(em)) > > goto out; > > } > > - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, > > - block_len, 0, > > - (1 << type) | > > - (1 << BTRFS_ORDERED_DIRECT), > > - BTRFS_COMPRESS_NONE); > > - if (ret) { > > + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, > > + block_start, block_len, 0, > > + (1 << type) | > > + (1 << BTRFS_ORDERED_DIRECT), > > + BTRFS_COMPRESS_NONE); > > + if (IS_ERR(ordered)) { > > if (em) { > > free_extent_map(em); > > btrfs_drop_extent_map_range(inode, start, > > start + len - 1, false); > > } > > - em = ERR_PTR(ret); > > + em = ERR_PTR(PTR_ERR(ordered)); > > + } else { > > + ASSERT(!dio_data->ordered); > > + dio_data->ordered = ordered; > > } > > out: > > > > @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > u64 start, u64 len) > > { > > struct btrfs_root *root = inode->root; > > @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > if (ret) > > return ERR_PTR(ret); > > > > - em = btrfs_create_dio_extent(inode, start, ins.offset, start, > > + em = btrfs_create_dio_extent(inode, dio_data, > > + start, ins.offset, start, > > ins.objectid, ins.offset, ins.offset, > > ins.offset, BTRFS_ORDERED_REGULAR); > > btrfs_dec_block_group_reservations(fs_info, ins.objectid); > > @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > } > > space_reserved = true; > > > > - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, > > + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, > > + start, len, > > orig_start, block_start, > > len, orig_block_len, > > ram_bytes, type); > > @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > goto out; > > space_reserved = true; > > > > - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); > > + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); > > if (IS_ERR(em)) { > > ret = PTR_ERR(em); > > goto out; > > @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > } > > } > > > > + if (dio_data->ordered) { > > + ASSERT(write); > > + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, > > + dio_data->ordered->file_offset, > > + dio_data->ordered->bytes_left); > > + if (IS_ERR(em)) { > > + ret = PTR_ERR(em); > > + goto err; > > + } > > + goto map_iomap; > > + } > > memset(dio_data, 0, sizeof(*dio_data)); > > > > /* > > @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > else > > free_extent_state(cached_state); > > > > +map_iomap: > > /* > > * Translate extent map information to iomap. > > * We trim the extents (and move the addr) even though iomap code does > > @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, > > if (submitted < length) { > > pos += submitted; > > length -= submitted; > > - if (write) > > - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, > > - pos, length, false); > > - else > > + if (write) { > > + if (submitted == 0) { > > + btrfs_mark_ordered_io_finished(BTRFS_I(inode), > > + NULL, pos, > > + length, false); > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > + } else { > > unlock_extent(&BTRFS_I(inode)->io_tree, pos, > > pos + length - 1, NULL); > > + } > > ret = -ENOTBLK; > > + } else { > > + /* On the last bio, release our cached ordered_extent */ > > + if (write) { > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > } > > > > if (write) > > @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { > > > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data data = { }; > > Btw, everywhere else we use the { 0 } style, so we should, ideally, be > consistent and use it here too. > > Those are just minor things, David can fix them up when he picks the > patch if he wants to, so: > > Reviewed-by: Filipe Manana <fdmanana@suse.com> And btw, as Christoph had asked before, can we get a test case for fstests with this scenario? Create a file (2M, close to the user's reproducer), mmap it to a buffer, then do a direct write into another file using that mmap'ed buffer - sounds like it should trigger the bug. Thanks. > > > > > return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > IOMAP_DIO_PARTIAL, &data, done_before); > > } > > > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; > > + struct iomap_dio *dio; > > > > - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > - IOMAP_DIO_PARTIAL, &data, done_before); > > + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > + IOMAP_DIO_PARTIAL, &dio_data, done_before); > > + if (!IS_ERR_OR_NULL(dio)) > > + *ordered_extent = dio_data.ordered; > > + return dio; > > } > > > > static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > > -- > > 2.38.1 > >
Looks good:
Reviewed-by: Christoph Hellwig <hch@lst.de>
On Wed, Feb 22, 2023 at 11:51:44AM +0000, Filipe Manana wrote: > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data data = { }; > > Btw, everywhere else we use the { 0 } style, so we should, ideally, be > consistent and use it here too. The empty initializer is just a newer C feature that hasn't caught on everywhere yet. It has the advantage of not creating a compile failure when a new first member gets added that can't be assigned to (or a sparse warning when it is a pointer). It has not downsides over the 0 initializer and should be used everywhere eventually.
On Wed, Feb 22, 2023 at 06:39:19AM -0800, Christoph Hellwig wrote: > On Wed, Feb 22, 2023 at 11:51:44AM +0000, Filipe Manana wrote: > > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > > > { > > > - struct btrfs_dio_data data; > > > + struct btrfs_dio_data data = { }; > > > > Btw, everywhere else we use the { 0 } style, so we should, ideally, be > > consistent and use it here too. > > The empty initializer is just a newer C feature that hasn't caught on > everywhere yet. It has the advantage of not creating a compile failure > when a new first member gets added that can't be assigned to (or > a sparse warning when it is a pointer). It has not downsides over > the 0 initializer and should be used everywhere eventually. We almost did that namely due to gcc 4.9 producing incomplete initialization with { 0 } but meanwhile the minimum version has been increased to 5.1 [1] and we've settled on { 0 } as it's the most common initiazlier. [1] https://lore.kernel.org/linux-btrfs/20220208170044.GD12643@twin.jikos.cz/
Hi, fstests generic/276 trigger a deadloop with the patch in misc-next. sysrq(w) output: [33812.455122] sysrq: Show Blocked State [33812.458831] task:kworker/u20:31 state:D stack:0 pid:1457865 ppid:2 flags:0x00004000 [33812.467371] Workqueue: btrfs-flush_delalloc btrfs_work_helper [btrfs] [33812.473916] Call Trace: [33812.476365] <TASK> [33812.478469] __schedule+0x2cb/0x880 [33812.481965] schedule+0x50/0xc0 [33812.485123] btrfs_start_ordered_extent+0xf7/0x140 [btrfs] [33812.490668] ? add_wait_queue+0xa0/0xa0 [33812.494508] btrfs_run_ordered_extent_work+0x1a/0x30 [btrfs] [33812.500219] btrfs_work_helper+0x27f/0x360 [btrfs] [33812.505073] process_one_work+0x1b0/0x390 [33812.509101] worker_thread+0x3c/0x370 [33812.512779] ? process_one_work+0x390/0x390 [33812.516979] kthread+0xe3/0x110 [33812.520138] ? kthread_complete_and_exit+0x20/0x20 [33812.524945] ret_from_fork+0x1f/0x30 [33812.528542] </TASK> [33812.530751] task:umount state:D stack:0 pid:1465218 ppid:1464868 flags:0x00004002 [33812.539372] Call Trace: [33812.541823] <TASK> [33812.543938] __schedule+0x2cb/0x880 [33812.547441] ? wait_for_completion+0x83/0x160 [33812.551807] schedule+0x50/0xc0 [33812.554964] schedule_timeout+0x269/0x310 [33812.559005] ? wait_for_completion+0x83/0x160 [33812.563370] wait_for_completion+0xb5/0x160 [33812.567561] btrfs_wait_ordered_extents+0x346/0x450 [btrfs] [33812.573197] btrfs_wait_ordered_roots+0x163/0x250 [btrfs] [33812.578645] btrfs_sync_fs+0x3e/0x1c0 [btrfs] [33812.583052] sync_filesystem+0x74/0x90 [33812.586830] generic_shutdown_super+0x22/0x120 [33812.591281] kill_anon_super+0x14/0x30 [33812.595049] btrfs_kill_super+0x12/0x20 [btrfs] [33812.599629] deactivate_locked_super+0x2c/0x70 [33812.604079] cleanup_mnt+0xb8/0x140 [33812.607585] task_work_run+0x6a/0xb0 [33812.611168] exit_to_user_mode_prepare+0x1b9/0x1c0 [33812.615977] syscall_exit_to_user_mode+0x12/0x30 [33812.620617] do_syscall_64+0x67/0x80 [33812.624198] ? syscall_exit_to_user_mode+0x12/0x30 [33812.629007] ? do_syscall_64+0x67/0x80 [33812.632771] ? syscall_exit_to_user_mode+0x12/0x30 [33812.637579] ? do_syscall_64+0x67/0x80 [33812.641346] ? syscall_exit_to_user_mode+0x12/0x30 [33812.646144] ? do_syscall_64+0x67/0x80 [33812.649907] ? exc_page_fault+0x64/0x140 [33812.653847] entry_SYSCALL_64_after_hwframe+0x63/0xcd [33812.658932] RIP: 0033:0x7f2eb454e56b [33812.662510] RSP: 002b:00007ffdf683ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [33812.670088] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f2eb454e56b [33812.677242] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000563731e5b5a0 [33812.684394] RBP: 0000563731e5b370 R08: 0000000000000000 R09: 00007ffdf6839a00 [33812.691536] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 [33812.698678] R13: 0000563731e5b5a0 R14: 0000563731e5b480 R15: 0000563731e5b370 [33812.705819] </TASK> Best Regards Wang Yugui (wangyugui@e16-tech.com) 2023/03/07 > If an application is doing direct io to a btrfs file and experiences a > page fault reading from the write buffer, iomap will issue a partial > bio, and allow the fs to keep going. However, there was a subtle bug in > this codepath in the btrfs dio iomap implementation that led to the > partial write ending up as a gap in the file's extents and to be read > back as zeros. > > The sequence of events in a partial write, lightly summarized and > trimmed down for brevity is as follows: > > ====WRITING TASK==== > btrfs_direct_write > __iomap_dio_write > iomap_iter > btrfs_dio_iomap_begin # create full ordered extent > iomap_dio_bio_iter > bio_iov_iter_get_pages # page fault; partial read > submit_bio # partial bio > iomap_iter > btrfs_dio_iomap_end > btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; > # submit to finish_ordered_fn wq > fault_in_iov_iter_readable # btrfs_direct_write detects partial write > __iomap_dio_write > iomap_iter > btrfs_dio_iomap_begin # create second partial ordered extent > iomap_dio_bio_iter > bio_iov_iter_get_pages # read all of remainder > submit_bio # partial bio with all of remainder > iomap_iter > btrfs_dio_iomap_end # nothing exciting to do with ordered io > > ====DIO ENDIO==== > ==FIRST PARTIAL BIO== > btrfs_dio_end_io > btrfs_mark_ordered_io_finished # bytes_left > 0 > # don't submit to finish_ordered_fn wq > ==SECOND PARTIAL BIO== > btrfs_dio_end_io > btrfs_mark_ordered_io_finished # bytes_left == 0 > # submit to finish_ordered_fn wq > > ====BTRFS FINISH ORDERED WQ==== > ==FIRST PARTIAL BIO== > btrfs_finish_ordered_io # called by dio_iomap_end_io, sees > # BTRFS_ORDERED_IOERR, just drops the > # ordered_extent > ==SECOND PARTIAL BIO== > btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file > # extents, csums, etc... > > The essence of the problem is that while btrfs_direct_write and iomap > properly interact to submit all the correct bios, there is insufficient > logic in the btrfs dio functions (btrfs_dio_iomap_begin, > btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to > ensure that every bio is at least a part of a completed ordered_extent. > And it is completing an ordered_extent that results in crucial > functionality like writing out a file extent for the range. > > More specifically, btrfs_dio_end_io treats the ordered extent as > unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. > Thus, the finish io work doesn't result in file extents, csums, etc... > In the aftermath, such a file behaves as though it has a hole in it, > instead of the purportedly written data. > > We considered a few options for fixing the bug (apologies for any > incorrect summary of a proposal which I didn't implement and fully > understand): > 1. treat the partial bio as if we had truncated the file, which would > result in properly finishing it. > 2. split the ordered extent when submitting a partial bio. > 3. cache the ordered extent across calls to __iomap_dio_rw in > iter->private, so that we could reuse it and correctly apply several > bios to it. > > I had trouble with 1, and it felt the most like a hack, so I tried 2 > and 3. Since 3 has the benefit of also not creating an extra file > extent, and avoids an ordered extent lookup during bio submission, it > felt like the best option. > > A quick summary of the changes necessary to implement this cached > ordered_extent behavior: > - btrfs_direct_write keeps track of an ordered_extent for the duration > of a call, possible across several __iomap_dio_rws. > - zero the btrfs_dio_data before using it, since its fields constitute > state now. > - btrfs_dio_write uses dio_data to pass this ordered extent into and out > of __iomap_dio_rw. > - when the write is done, put the ordered_extent. > - if the short write happens to be length 0, then we _don't_ get an > extra bio, so we do need to cancel the ordered_extent like we used > to (and ditch the cached ordered extent) > - in btrfs_dio_iomap_begin, if the cached ordered extent is present, > skip all the work of creating it, just look up the extent mapping and > jump to setting up the iomap. (This part could likely be more > elegant..) > > Thanks to Josef, Christoph, and Filipe with their help figuring out the > bug and the fix. > > Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 > Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ > Link: https://pastebin.com/3SDaH8C6 > Signed-off-by: Boris Burkov <boris@bur.io> > --- > fs/btrfs/btrfs_inode.h | 1 + > fs/btrfs/file.c | 11 ++++++- > fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- > 3 files changed, 67 insertions(+), 20 deletions(-) > > diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h > index 49a92aa65de1..87020aa58121 100644 > --- a/fs/btrfs/btrfs_inode.h > +++ b/fs/btrfs/btrfs_inode.h > @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, > size_t done_before); > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > + struct btrfs_ordered_extent **ordered_extent, > size_t done_before); > > extern const struct dentry_operations btrfs_dentry_operations; > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > index 5cc5a1faaef5..ec5c5355906b 100644 > --- a/fs/btrfs/file.c > +++ b/fs/btrfs/file.c > @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > ssize_t err; > unsigned int ilock_flags = 0; > struct iomap_dio *dio; > + struct btrfs_ordered_extent *ordered_extent = NULL; > > if (iocb->ki_flags & IOCB_NOWAIT) > ilock_flags |= BTRFS_ILOCK_TRY; > @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > * got -EFAULT, faulting in the pages before the retry. > */ > from->nofault = true; > - dio = btrfs_dio_write(iocb, from, written); > + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); > from->nofault = false; > > /* > @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > goto relock; > } > } > + /* > + * We can't loop back to btrfs_dio_write, so we can drop the cached > + * ordered extent. Typically btrfs_dio_iomap_end will run and put the > + * ordered_extent, but this is needed to clean up in case of an error > + * path breaking out of iomap_iter before the final iomap_end call. > + */ > + if (ordered_extent) > + btrfs_put_ordered_extent(ordered_extent); > > /* > * If 'err' is -ENOTBLK or we have not written all data, then it means > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > index 44e9acc77a74..f1a59c5f3140 100644 > --- a/fs/btrfs/inode.c > +++ b/fs/btrfs/inode.c > @@ -81,6 +81,7 @@ struct btrfs_dio_data { > struct extent_changeset *data_reserved; > bool data_space_reserved; > bool nocow_done; > + struct btrfs_ordered_extent *ordered; > }; > > struct btrfs_dio_private { > @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, > } > > static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > + struct btrfs_dio_data *dio_data, > const u64 start, > const u64 len, > const u64 orig_start, > @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > const int type) > { > struct extent_map *em = NULL; > - int ret; > + struct btrfs_ordered_extent *ordered; > > if (type != BTRFS_ORDERED_NOCOW) { > em = create_io_em(inode, start, len, orig_start, block_start, > @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > if (IS_ERR(em)) > goto out; > } > - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, > - block_len, 0, > - (1 << type) | > - (1 << BTRFS_ORDERED_DIRECT), > - BTRFS_COMPRESS_NONE); > - if (ret) { > + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, > + block_start, block_len, 0, > + (1 << type) | > + (1 << BTRFS_ORDERED_DIRECT), > + BTRFS_COMPRESS_NONE); > + if (IS_ERR(ordered)) { > if (em) { > free_extent_map(em); > btrfs_drop_extent_map_range(inode, start, > start + len - 1, false); > } > - em = ERR_PTR(ret); > + em = ERR_PTR(PTR_ERR(ordered)); > + } else { > + ASSERT(!dio_data->ordered); > + dio_data->ordered = ordered; > } > out: > > @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > } > > static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > + struct btrfs_dio_data *dio_data, > u64 start, u64 len) > { > struct btrfs_root *root = inode->root; > @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > if (ret) > return ERR_PTR(ret); > > - em = btrfs_create_dio_extent(inode, start, ins.offset, start, > + em = btrfs_create_dio_extent(inode, dio_data, > + start, ins.offset, start, > ins.objectid, ins.offset, ins.offset, > ins.offset, BTRFS_ORDERED_REGULAR); > btrfs_dec_block_group_reservations(fs_info, ins.objectid); > @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > } > space_reserved = true; > > - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, > + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, > + start, len, > orig_start, block_start, > len, orig_block_len, > ram_bytes, type); > @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > goto out; > space_reserved = true; > > - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); > + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); > if (IS_ERR(em)) { > ret = PTR_ERR(em); > goto out; > @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > } > } > > + if (dio_data->ordered) { > + ASSERT(write); > + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, > + dio_data->ordered->file_offset, > + dio_data->ordered->bytes_left); > + if (IS_ERR(em)) { > + ret = PTR_ERR(em); > + goto err; > + } > + goto map_iomap; > + } > memset(dio_data, 0, sizeof(*dio_data)); > > /* > @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > else > free_extent_state(cached_state); > > +map_iomap: > /* > * Translate extent map information to iomap. > * We trim the extents (and move the addr) even though iomap code does > @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, > if (submitted < length) { > pos += submitted; > length -= submitted; > - if (write) > - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, > - pos, length, false); > - else > + if (write) { > + if (submitted == 0) { > + btrfs_mark_ordered_io_finished(BTRFS_I(inode), > + NULL, pos, > + length, false); > + btrfs_put_ordered_extent(dio_data->ordered); > + dio_data->ordered = NULL; > + } > + } else { > unlock_extent(&BTRFS_I(inode)->io_tree, pos, > pos + length - 1, NULL); > + } > ret = -ENOTBLK; > + } else { > + /* On the last bio, release our cached ordered_extent */ > + if (write) { > + btrfs_put_ordered_extent(dio_data->ordered); > + dio_data->ordered = NULL; > + } > } > > if (write) > @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > { > - struct btrfs_dio_data data; > + struct btrfs_dio_data data = { }; > > return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > IOMAP_DIO_PARTIAL, &data, done_before); > } > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > + struct btrfs_ordered_extent **ordered_extent, > size_t done_before) > { > - struct btrfs_dio_data data; > + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; > + struct iomap_dio *dio; > > - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > - IOMAP_DIO_PARTIAL, &data, done_before); > + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > + IOMAP_DIO_PARTIAL, &dio_data, done_before); > + if (!IS_ERR_OR_NULL(dio)) > + *ordered_extent = dio_data.ordered; > + return dio; > } > > static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > -- > 2.38.1
On Tue, Mar 07, 2023 at 07:07:21AM +0800, Wang Yugui wrote: > Hi, > > fstests generic/276 trigger a deadloop with the patch in misc-next. > > sysrq(w) output: > > [33812.455122] sysrq: Show Blocked State > > [33812.458831] task:kworker/u20:31 state:D stack:0 pid:1457865 ppid:2 flags:0x00004000 > [33812.467371] Workqueue: btrfs-flush_delalloc btrfs_work_helper [btrfs] > [33812.473916] Call Trace: > [33812.476365] <TASK> > [33812.478469] __schedule+0x2cb/0x880 > [33812.481965] schedule+0x50/0xc0 > [33812.485123] btrfs_start_ordered_extent+0xf7/0x140 [btrfs] > [33812.490668] ? add_wait_queue+0xa0/0xa0 > [33812.494508] btrfs_run_ordered_extent_work+0x1a/0x30 [btrfs] > [33812.500219] btrfs_work_helper+0x27f/0x360 [btrfs] > [33812.505073] process_one_work+0x1b0/0x390 > [33812.509101] worker_thread+0x3c/0x370 > [33812.512779] ? process_one_work+0x390/0x390 > [33812.516979] kthread+0xe3/0x110 > [33812.520138] ? kthread_complete_and_exit+0x20/0x20 > [33812.524945] ret_from_fork+0x1f/0x30 > [33812.528542] </TASK> > > [33812.530751] task:umount state:D stack:0 pid:1465218 ppid:1464868 flags:0x00004002 > [33812.539372] Call Trace: > [33812.541823] <TASK> > [33812.543938] __schedule+0x2cb/0x880 > [33812.547441] ? wait_for_completion+0x83/0x160 > [33812.551807] schedule+0x50/0xc0 > [33812.554964] schedule_timeout+0x269/0x310 > [33812.559005] ? wait_for_completion+0x83/0x160 > [33812.563370] wait_for_completion+0xb5/0x160 > [33812.567561] btrfs_wait_ordered_extents+0x346/0x450 [btrfs] > [33812.573197] btrfs_wait_ordered_roots+0x163/0x250 [btrfs] > [33812.578645] btrfs_sync_fs+0x3e/0x1c0 [btrfs] > [33812.583052] sync_filesystem+0x74/0x90 > [33812.586830] generic_shutdown_super+0x22/0x120 > [33812.591281] kill_anon_super+0x14/0x30 > [33812.595049] btrfs_kill_super+0x12/0x20 [btrfs] > [33812.599629] deactivate_locked_super+0x2c/0x70 > [33812.604079] cleanup_mnt+0xb8/0x140 > [33812.607585] task_work_run+0x6a/0xb0 > [33812.611168] exit_to_user_mode_prepare+0x1b9/0x1c0 > [33812.615977] syscall_exit_to_user_mode+0x12/0x30 > [33812.620617] do_syscall_64+0x67/0x80 > [33812.624198] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.629007] ? do_syscall_64+0x67/0x80 > [33812.632771] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.637579] ? do_syscall_64+0x67/0x80 > [33812.641346] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.646144] ? do_syscall_64+0x67/0x80 > [33812.649907] ? exc_page_fault+0x64/0x140 > [33812.653847] entry_SYSCALL_64_after_hwframe+0x63/0xcd > [33812.658932] RIP: 0033:0x7f2eb454e56b > [33812.662510] RSP: 002b:00007ffdf683ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 > [33812.670088] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f2eb454e56b > [33812.677242] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000563731e5b5a0 > [33812.684394] RBP: 0000563731e5b370 R08: 0000000000000000 R09: 00007ffdf6839a00 > [33812.691536] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 > [33812.698678] R13: 0000563731e5b5a0 R14: 0000563731e5b480 R15: 0000563731e5b370 > [33812.705819] </TASK> > > > Best Regards > Wang Yugui (wangyugui@e16-tech.com) > 2023/03/07 Thanks for the report. We have seen a similar regression on a similar test (generic/250) on most test machines except unfortunately mine. I have an idea for a fix, but it's hard to hack on it without a repro :( Hopefully should have a fix today or tomorrow. Boris > > > If an application is doing direct io to a btrfs file and experiences a > > page fault reading from the write buffer, iomap will issue a partial > > bio, and allow the fs to keep going. However, there was a subtle bug in > > this codepath in the btrfs dio iomap implementation that led to the > > partial write ending up as a gap in the file's extents and to be read > > back as zeros. > > > > The sequence of events in a partial write, lightly summarized and > > trimmed down for brevity is as follows: > > > > ====WRITING TASK==== > > btrfs_direct_write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create full ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # page fault; partial read > > submit_bio # partial bio > > iomap_iter > > btrfs_dio_iomap_end > > btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; > > # submit to finish_ordered_fn wq > > fault_in_iov_iter_readable # btrfs_direct_write detects partial write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create second partial ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # read all of remainder > > submit_bio # partial bio with all of remainder > > iomap_iter > > btrfs_dio_iomap_end # nothing exciting to do with ordered io > > > > ====DIO ENDIO==== > > ==FIRST PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left > 0 > > # don't submit to finish_ordered_fn wq > > ==SECOND PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left == 0 > > # submit to finish_ordered_fn wq > > > > ====BTRFS FINISH ORDERED WQ==== > > ==FIRST PARTIAL BIO== > > btrfs_finish_ordered_io # called by dio_iomap_end_io, sees > > # BTRFS_ORDERED_IOERR, just drops the > > # ordered_extent > > ==SECOND PARTIAL BIO== > > btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file > > # extents, csums, etc... > > > > The essence of the problem is that while btrfs_direct_write and iomap > > properly interact to submit all the correct bios, there is insufficient > > logic in the btrfs dio functions (btrfs_dio_iomap_begin, > > btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to > > ensure that every bio is at least a part of a completed ordered_extent. > > And it is completing an ordered_extent that results in crucial > > functionality like writing out a file extent for the range. > > > > More specifically, btrfs_dio_end_io treats the ordered extent as > > unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. > > Thus, the finish io work doesn't result in file extents, csums, etc... > > In the aftermath, such a file behaves as though it has a hole in it, > > instead of the purportedly written data. > > > > We considered a few options for fixing the bug (apologies for any > > incorrect summary of a proposal which I didn't implement and fully > > understand): > > 1. treat the partial bio as if we had truncated the file, which would > > result in properly finishing it. > > 2. split the ordered extent when submitting a partial bio. > > 3. cache the ordered extent across calls to __iomap_dio_rw in > > iter->private, so that we could reuse it and correctly apply several > > bios to it. > > > > I had trouble with 1, and it felt the most like a hack, so I tried 2 > > and 3. Since 3 has the benefit of also not creating an extra file > > extent, and avoids an ordered extent lookup during bio submission, it > > felt like the best option. > > > > A quick summary of the changes necessary to implement this cached > > ordered_extent behavior: > > - btrfs_direct_write keeps track of an ordered_extent for the duration > > of a call, possible across several __iomap_dio_rws. > > - zero the btrfs_dio_data before using it, since its fields constitute > > state now. > > - btrfs_dio_write uses dio_data to pass this ordered extent into and out > > of __iomap_dio_rw. > > - when the write is done, put the ordered_extent. > > - if the short write happens to be length 0, then we _don't_ get an > > extra bio, so we do need to cancel the ordered_extent like we used > > to (and ditch the cached ordered extent) > > - in btrfs_dio_iomap_begin, if the cached ordered extent is present, > > skip all the work of creating it, just look up the extent mapping and > > jump to setting up the iomap. (This part could likely be more > > elegant..) > > > > Thanks to Josef, Christoph, and Filipe with their help figuring out the > > bug and the fix. > > > > Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 > > Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ > > Link: https://pastebin.com/3SDaH8C6 > > Signed-off-by: Boris Burkov <boris@bur.io> > > --- > > fs/btrfs/btrfs_inode.h | 1 + > > fs/btrfs/file.c | 11 ++++++- > > fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- > > 3 files changed, 67 insertions(+), 20 deletions(-) > > > > diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h > > index 49a92aa65de1..87020aa58121 100644 > > --- a/fs/btrfs/btrfs_inode.h > > +++ b/fs/btrfs/btrfs_inode.h > > @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, > > size_t done_before); > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before); > > > > extern const struct dentry_operations btrfs_dentry_operations; > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > > index 5cc5a1faaef5..ec5c5355906b 100644 > > --- a/fs/btrfs/file.c > > +++ b/fs/btrfs/file.c > > @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > ssize_t err; > > unsigned int ilock_flags = 0; > > struct iomap_dio *dio; > > + struct btrfs_ordered_extent *ordered_extent = NULL; > > > > if (iocb->ki_flags & IOCB_NOWAIT) > > ilock_flags |= BTRFS_ILOCK_TRY; > > @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > * got -EFAULT, faulting in the pages before the retry. > > */ > > from->nofault = true; > > - dio = btrfs_dio_write(iocb, from, written); > > + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); > > from->nofault = false; > > > > /* > > @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > goto relock; > > } > > } > > + /* > > + * We can't loop back to btrfs_dio_write, so we can drop the cached > > + * ordered extent. Typically btrfs_dio_iomap_end will run and put the > > + * ordered_extent, but this is needed to clean up in case of an error > > + * path breaking out of iomap_iter before the final iomap_end call. > > + */ > > + if (ordered_extent) > > + btrfs_put_ordered_extent(ordered_extent); > > > > /* > > * If 'err' is -ENOTBLK or we have not written all data, then it means > > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > > index 44e9acc77a74..f1a59c5f3140 100644 > > --- a/fs/btrfs/inode.c > > +++ b/fs/btrfs/inode.c > > @@ -81,6 +81,7 @@ struct btrfs_dio_data { > > struct extent_changeset *data_reserved; > > bool data_space_reserved; > > bool nocow_done; > > + struct btrfs_ordered_extent *ordered; > > }; > > > > struct btrfs_dio_private { > > @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > const u64 start, > > const u64 len, > > const u64 orig_start, > > @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > const int type) > > { > > struct extent_map *em = NULL; > > - int ret; > > + struct btrfs_ordered_extent *ordered; > > > > if (type != BTRFS_ORDERED_NOCOW) { > > em = create_io_em(inode, start, len, orig_start, block_start, > > @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > if (IS_ERR(em)) > > goto out; > > } > > - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, > > - block_len, 0, > > - (1 << type) | > > - (1 << BTRFS_ORDERED_DIRECT), > > - BTRFS_COMPRESS_NONE); > > - if (ret) { > > + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, > > + block_start, block_len, 0, > > + (1 << type) | > > + (1 << BTRFS_ORDERED_DIRECT), > > + BTRFS_COMPRESS_NONE); > > + if (IS_ERR(ordered)) { > > if (em) { > > free_extent_map(em); > > btrfs_drop_extent_map_range(inode, start, > > start + len - 1, false); > > } > > - em = ERR_PTR(ret); > > + em = ERR_PTR(PTR_ERR(ordered)); > > + } else { > > + ASSERT(!dio_data->ordered); > > + dio_data->ordered = ordered; > > } > > out: > > > > @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > u64 start, u64 len) > > { > > struct btrfs_root *root = inode->root; > > @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > if (ret) > > return ERR_PTR(ret); > > > > - em = btrfs_create_dio_extent(inode, start, ins.offset, start, > > + em = btrfs_create_dio_extent(inode, dio_data, > > + start, ins.offset, start, > > ins.objectid, ins.offset, ins.offset, > > ins.offset, BTRFS_ORDERED_REGULAR); > > btrfs_dec_block_group_reservations(fs_info, ins.objectid); > > @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > } > > space_reserved = true; > > > > - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, > > + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, > > + start, len, > > orig_start, block_start, > > len, orig_block_len, > > ram_bytes, type); > > @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > goto out; > > space_reserved = true; > > > > - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); > > + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); > > if (IS_ERR(em)) { > > ret = PTR_ERR(em); > > goto out; > > @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > } > > } > > > > + if (dio_data->ordered) { > > + ASSERT(write); > > + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, > > + dio_data->ordered->file_offset, > > + dio_data->ordered->bytes_left); > > + if (IS_ERR(em)) { > > + ret = PTR_ERR(em); > > + goto err; > > + } > > + goto map_iomap; > > + } > > memset(dio_data, 0, sizeof(*dio_data)); > > > > /* > > @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > else > > free_extent_state(cached_state); > > > > +map_iomap: > > /* > > * Translate extent map information to iomap. > > * We trim the extents (and move the addr) even though iomap code does > > @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, > > if (submitted < length) { > > pos += submitted; > > length -= submitted; > > - if (write) > > - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, > > - pos, length, false); > > - else > > + if (write) { > > + if (submitted == 0) { > > + btrfs_mark_ordered_io_finished(BTRFS_I(inode), > > + NULL, pos, > > + length, false); > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > + } else { > > unlock_extent(&BTRFS_I(inode)->io_tree, pos, > > pos + length - 1, NULL); > > + } > > ret = -ENOTBLK; > > + } else { > > + /* On the last bio, release our cached ordered_extent */ > > + if (write) { > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > } > > > > if (write) > > @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { > > > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data data = { }; > > > > return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > IOMAP_DIO_PARTIAL, &data, done_before); > > } > > > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; > > + struct iomap_dio *dio; > > > > - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > - IOMAP_DIO_PARTIAL, &data, done_before); > > + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > + IOMAP_DIO_PARTIAL, &dio_data, done_before); > > + if (!IS_ERR_OR_NULL(dio)) > > + *ordered_extent = dio_data.ordered; > > + return dio; > > } > > > > static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > > -- > > 2.38.1 > >
On Tue, Mar 07, 2023 at 07:07:21AM +0800, Wang Yugui wrote: > Hi, > > fstests generic/276 trigger a deadloop with the patch in misc-next. > > sysrq(w) output: > > [33812.455122] sysrq: Show Blocked State > > [33812.458831] task:kworker/u20:31 state:D stack:0 pid:1457865 ppid:2 flags:0x00004000 > [33812.467371] Workqueue: btrfs-flush_delalloc btrfs_work_helper [btrfs] > [33812.473916] Call Trace: > [33812.476365] <TASK> > [33812.478469] __schedule+0x2cb/0x880 > [33812.481965] schedule+0x50/0xc0 > [33812.485123] btrfs_start_ordered_extent+0xf7/0x140 [btrfs] > [33812.490668] ? add_wait_queue+0xa0/0xa0 > [33812.494508] btrfs_run_ordered_extent_work+0x1a/0x30 [btrfs] > [33812.500219] btrfs_work_helper+0x27f/0x360 [btrfs] > [33812.505073] process_one_work+0x1b0/0x390 > [33812.509101] worker_thread+0x3c/0x370 > [33812.512779] ? process_one_work+0x390/0x390 > [33812.516979] kthread+0xe3/0x110 > [33812.520138] ? kthread_complete_and_exit+0x20/0x20 > [33812.524945] ret_from_fork+0x1f/0x30 > [33812.528542] </TASK> > > [33812.530751] task:umount state:D stack:0 pid:1465218 ppid:1464868 flags:0x00004002 > [33812.539372] Call Trace: > [33812.541823] <TASK> > [33812.543938] __schedule+0x2cb/0x880 > [33812.547441] ? wait_for_completion+0x83/0x160 > [33812.551807] schedule+0x50/0xc0 > [33812.554964] schedule_timeout+0x269/0x310 > [33812.559005] ? wait_for_completion+0x83/0x160 > [33812.563370] wait_for_completion+0xb5/0x160 > [33812.567561] btrfs_wait_ordered_extents+0x346/0x450 [btrfs] > [33812.573197] btrfs_wait_ordered_roots+0x163/0x250 [btrfs] > [33812.578645] btrfs_sync_fs+0x3e/0x1c0 [btrfs] > [33812.583052] sync_filesystem+0x74/0x90 > [33812.586830] generic_shutdown_super+0x22/0x120 > [33812.591281] kill_anon_super+0x14/0x30 > [33812.595049] btrfs_kill_super+0x12/0x20 [btrfs] > [33812.599629] deactivate_locked_super+0x2c/0x70 > [33812.604079] cleanup_mnt+0xb8/0x140 > [33812.607585] task_work_run+0x6a/0xb0 > [33812.611168] exit_to_user_mode_prepare+0x1b9/0x1c0 > [33812.615977] syscall_exit_to_user_mode+0x12/0x30 > [33812.620617] do_syscall_64+0x67/0x80 > [33812.624198] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.629007] ? do_syscall_64+0x67/0x80 > [33812.632771] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.637579] ? do_syscall_64+0x67/0x80 > [33812.641346] ? syscall_exit_to_user_mode+0x12/0x30 > [33812.646144] ? do_syscall_64+0x67/0x80 > [33812.649907] ? exc_page_fault+0x64/0x140 > [33812.653847] entry_SYSCALL_64_after_hwframe+0x63/0xcd > [33812.658932] RIP: 0033:0x7f2eb454e56b > [33812.662510] RSP: 002b:00007ffdf683ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 > [33812.670088] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f2eb454e56b > [33812.677242] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000563731e5b5a0 > [33812.684394] RBP: 0000563731e5b370 R08: 0000000000000000 R09: 00007ffdf6839a00 > [33812.691536] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 > [33812.698678] R13: 0000563731e5b5a0 R14: 0000563731e5b480 R15: 0000563731e5b370 > [33812.705819] </TASK> > > > Best Regards > Wang Yugui (wangyugui@e16-tech.com) > 2023/03/07 I sent a V3 that fixed our issue on generic/250. Since I didn't have a reproducer for this issue, I was not able to test it. But the tests are somewhat similar (dmerror + dio) as are the symptoms (stuck on ordered extents) so I suspect they were hitting the same bug. If you have a chance, would you be able to apply the V3 and test it out again on your system? Thanks, Boris > > > If an application is doing direct io to a btrfs file and experiences a > > page fault reading from the write buffer, iomap will issue a partial > > bio, and allow the fs to keep going. However, there was a subtle bug in > > this codepath in the btrfs dio iomap implementation that led to the > > partial write ending up as a gap in the file's extents and to be read > > back as zeros. > > > > The sequence of events in a partial write, lightly summarized and > > trimmed down for brevity is as follows: > > > > ====WRITING TASK==== > > btrfs_direct_write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create full ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # page fault; partial read > > submit_bio # partial bio > > iomap_iter > > btrfs_dio_iomap_end > > btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; > > # submit to finish_ordered_fn wq > > fault_in_iov_iter_readable # btrfs_direct_write detects partial write > > __iomap_dio_write > > iomap_iter > > btrfs_dio_iomap_begin # create second partial ordered extent > > iomap_dio_bio_iter > > bio_iov_iter_get_pages # read all of remainder > > submit_bio # partial bio with all of remainder > > iomap_iter > > btrfs_dio_iomap_end # nothing exciting to do with ordered io > > > > ====DIO ENDIO==== > > ==FIRST PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left > 0 > > # don't submit to finish_ordered_fn wq > > ==SECOND PARTIAL BIO== > > btrfs_dio_end_io > > btrfs_mark_ordered_io_finished # bytes_left == 0 > > # submit to finish_ordered_fn wq > > > > ====BTRFS FINISH ORDERED WQ==== > > ==FIRST PARTIAL BIO== > > btrfs_finish_ordered_io # called by dio_iomap_end_io, sees > > # BTRFS_ORDERED_IOERR, just drops the > > # ordered_extent > > ==SECOND PARTIAL BIO== > > btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file > > # extents, csums, etc... > > > > The essence of the problem is that while btrfs_direct_write and iomap > > properly interact to submit all the correct bios, there is insufficient > > logic in the btrfs dio functions (btrfs_dio_iomap_begin, > > btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to > > ensure that every bio is at least a part of a completed ordered_extent. > > And it is completing an ordered_extent that results in crucial > > functionality like writing out a file extent for the range. > > > > More specifically, btrfs_dio_end_io treats the ordered extent as > > unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. > > Thus, the finish io work doesn't result in file extents, csums, etc... > > In the aftermath, such a file behaves as though it has a hole in it, > > instead of the purportedly written data. > > > > We considered a few options for fixing the bug (apologies for any > > incorrect summary of a proposal which I didn't implement and fully > > understand): > > 1. treat the partial bio as if we had truncated the file, which would > > result in properly finishing it. > > 2. split the ordered extent when submitting a partial bio. > > 3. cache the ordered extent across calls to __iomap_dio_rw in > > iter->private, so that we could reuse it and correctly apply several > > bios to it. > > > > I had trouble with 1, and it felt the most like a hack, so I tried 2 > > and 3. Since 3 has the benefit of also not creating an extra file > > extent, and avoids an ordered extent lookup during bio submission, it > > felt like the best option. > > > > A quick summary of the changes necessary to implement this cached > > ordered_extent behavior: > > - btrfs_direct_write keeps track of an ordered_extent for the duration > > of a call, possible across several __iomap_dio_rws. > > - zero the btrfs_dio_data before using it, since its fields constitute > > state now. > > - btrfs_dio_write uses dio_data to pass this ordered extent into and out > > of __iomap_dio_rw. > > - when the write is done, put the ordered_extent. > > - if the short write happens to be length 0, then we _don't_ get an > > extra bio, so we do need to cancel the ordered_extent like we used > > to (and ditch the cached ordered extent) > > - in btrfs_dio_iomap_begin, if the cached ordered extent is present, > > skip all the work of creating it, just look up the extent mapping and > > jump to setting up the iomap. (This part could likely be more > > elegant..) > > > > Thanks to Josef, Christoph, and Filipe with their help figuring out the > > bug and the fix. > > > > Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 > > Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ > > Link: https://pastebin.com/3SDaH8C6 > > Signed-off-by: Boris Burkov <boris@bur.io> > > --- > > fs/btrfs/btrfs_inode.h | 1 + > > fs/btrfs/file.c | 11 ++++++- > > fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- > > 3 files changed, 67 insertions(+), 20 deletions(-) > > > > diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h > > index 49a92aa65de1..87020aa58121 100644 > > --- a/fs/btrfs/btrfs_inode.h > > +++ b/fs/btrfs/btrfs_inode.h > > @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, > > size_t done_before); > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before); > > > > extern const struct dentry_operations btrfs_dentry_operations; > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > > index 5cc5a1faaef5..ec5c5355906b 100644 > > --- a/fs/btrfs/file.c > > +++ b/fs/btrfs/file.c > > @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > ssize_t err; > > unsigned int ilock_flags = 0; > > struct iomap_dio *dio; > > + struct btrfs_ordered_extent *ordered_extent = NULL; > > > > if (iocb->ki_flags & IOCB_NOWAIT) > > ilock_flags |= BTRFS_ILOCK_TRY; > > @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > * got -EFAULT, faulting in the pages before the retry. > > */ > > from->nofault = true; > > - dio = btrfs_dio_write(iocb, from, written); > > + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); > > from->nofault = false; > > > > /* > > @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) > > goto relock; > > } > > } > > + /* > > + * We can't loop back to btrfs_dio_write, so we can drop the cached > > + * ordered extent. Typically btrfs_dio_iomap_end will run and put the > > + * ordered_extent, but this is needed to clean up in case of an error > > + * path breaking out of iomap_iter before the final iomap_end call. > > + */ > > + if (ordered_extent) > > + btrfs_put_ordered_extent(ordered_extent); > > > > /* > > * If 'err' is -ENOTBLK or we have not written all data, then it means > > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c > > index 44e9acc77a74..f1a59c5f3140 100644 > > --- a/fs/btrfs/inode.c > > +++ b/fs/btrfs/inode.c > > @@ -81,6 +81,7 @@ struct btrfs_dio_data { > > struct extent_changeset *data_reserved; > > bool data_space_reserved; > > bool nocow_done; > > + struct btrfs_ordered_extent *ordered; > > }; > > > > struct btrfs_dio_private { > > @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > const u64 start, > > const u64 len, > > const u64 orig_start, > > @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > const int type) > > { > > struct extent_map *em = NULL; > > - int ret; > > + struct btrfs_ordered_extent *ordered; > > > > if (type != BTRFS_ORDERED_NOCOW) { > > em = create_io_em(inode, start, len, orig_start, block_start, > > @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > if (IS_ERR(em)) > > goto out; > > } > > - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, > > - block_len, 0, > > - (1 << type) | > > - (1 << BTRFS_ORDERED_DIRECT), > > - BTRFS_COMPRESS_NONE); > > - if (ret) { > > + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, > > + block_start, block_len, 0, > > + (1 << type) | > > + (1 << BTRFS_ORDERED_DIRECT), > > + BTRFS_COMPRESS_NONE); > > + if (IS_ERR(ordered)) { > > if (em) { > > free_extent_map(em); > > btrfs_drop_extent_map_range(inode, start, > > start + len - 1, false); > > } > > - em = ERR_PTR(ret); > > + em = ERR_PTR(PTR_ERR(ordered)); > > + } else { > > + ASSERT(!dio_data->ordered); > > + dio_data->ordered = ordered; > > } > > out: > > > > @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, > > } > > > > static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > + struct btrfs_dio_data *dio_data, > > u64 start, u64 len) > > { > > struct btrfs_root *root = inode->root; > > @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, > > if (ret) > > return ERR_PTR(ret); > > > > - em = btrfs_create_dio_extent(inode, start, ins.offset, start, > > + em = btrfs_create_dio_extent(inode, dio_data, > > + start, ins.offset, start, > > ins.objectid, ins.offset, ins.offset, > > ins.offset, BTRFS_ORDERED_REGULAR); > > btrfs_dec_block_group_reservations(fs_info, ins.objectid); > > @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > } > > space_reserved = true; > > > > - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, > > + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, > > + start, len, > > orig_start, block_start, > > len, orig_block_len, > > ram_bytes, type); > > @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, > > goto out; > > space_reserved = true; > > > > - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); > > + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); > > if (IS_ERR(em)) { > > ret = PTR_ERR(em); > > goto out; > > @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > } > > } > > > > + if (dio_data->ordered) { > > + ASSERT(write); > > + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, > > + dio_data->ordered->file_offset, > > + dio_data->ordered->bytes_left); > > + if (IS_ERR(em)) { > > + ret = PTR_ERR(em); > > + goto err; > > + } > > + goto map_iomap; > > + } > > memset(dio_data, 0, sizeof(*dio_data)); > > > > /* > > @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, > > else > > free_extent_state(cached_state); > > > > +map_iomap: > > /* > > * Translate extent map information to iomap. > > * We trim the extents (and move the addr) even though iomap code does > > @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, > > if (submitted < length) { > > pos += submitted; > > length -= submitted; > > - if (write) > > - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, > > - pos, length, false); > > - else > > + if (write) { > > + if (submitted == 0) { > > + btrfs_mark_ordered_io_finished(BTRFS_I(inode), > > + NULL, pos, > > + length, false); > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > + } else { > > unlock_extent(&BTRFS_I(inode)->io_tree, pos, > > pos + length - 1, NULL); > > + } > > ret = -ENOTBLK; > > + } else { > > + /* On the last bio, release our cached ordered_extent */ > > + if (write) { > > + btrfs_put_ordered_extent(dio_data->ordered); > > + dio_data->ordered = NULL; > > + } > > } > > > > if (write) > > @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { > > > > ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data data = { }; > > > > return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > IOMAP_DIO_PARTIAL, &data, done_before); > > } > > > > struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, > > + struct btrfs_ordered_extent **ordered_extent, > > size_t done_before) > > { > > - struct btrfs_dio_data data; > > + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; > > + struct iomap_dio *dio; > > > > - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > - IOMAP_DIO_PARTIAL, &data, done_before); > > + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, > > + IOMAP_DIO_PARTIAL, &dio_data, done_before); > > + if (!IS_ERR_OR_NULL(dio)) > > + *ordered_extent = dio_data.ordered; > > + return dio; > > } > > > > static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > > -- > > 2.38.1 > >
I hopefully was able to reproduce your hang in generic/276. Is this
similar to your observed dmesg output before the sysrq output you pasted?
[ 393.840871] Buffer I/O error on dev dm-1, logical block 31457264,
async page read
[ 393.878325] BTRFS error (device dm-1): bdev /dev/mapper/error-test
errs: wr 1, rd 0, flush 0, corrupt 0, gen 0
[ 393.878335] BTRFS warning (device dm-1): direct IO failed ino 259 op
0x8801 offset 0x0 len 1982464 err no 10
[ 393.878712] BTRFS error (device dm-1): bdev /dev/mapper/error-test
errs: wr 2, rd 0, flush 0, corrupt 0, gen 0
[ 393.878715] BTRFS warning (device dm-1): direct IO failed ino 259 op
0x8801 offset 0x1e4000 len 2162688 err no 10
[ 393.924856] BTRFS info: devid 1 device path /dev/mapper/error-test
changed to /dev/dm-1 scanned by systemd-udevd (1598)
[ 393.926581] BTRFS info: devid 1 device path /dev/dm-1 changed to
/dev/mapper/error-test scanned by systemd-udevd (1598)
[ 868.210173] sysrq: Show Blocked State
[ 868.210869] task:kworker/u6:5 state:D stack:0 pid:96 ppid:2
flags:0x00004000
...
My generic/276 repro has the same form of error messages as for
generic/250, so hopefully they're both fixed by v3 of this patch;
testing with that new patch now.
On 3/6/23 18:59, Boris Burkov wrote:
> On Tue, Mar 07, 2023 at 07:07:21AM +0800, Wang Yugui wrote:
>> Hi,
>>
>> fstests generic/276 trigger a deadloop with the patch in misc-next.
>>
>> sysrq(w) output:
>>
>> [33812.455122] sysrq: Show Blocked State
>>
>> [33812.458831] task:kworker/u20:31 state:D stack:0 pid:1457865 ppid:2 flags:0x00004000
>> [33812.467371] Workqueue: btrfs-flush_delalloc btrfs_work_helper [btrfs]
>> [33812.473916] Call Trace:
>> [33812.476365] <TASK>
>> [33812.478469] __schedule+0x2cb/0x880
>> [33812.481965] schedule+0x50/0xc0
>> [33812.485123] btrfs_start_ordered_extent+0xf7/0x140 [btrfs]
>> [33812.490668] ? add_wait_queue+0xa0/0xa0
>> [33812.494508] btrfs_run_ordered_extent_work+0x1a/0x30 [btrfs]
>> [33812.500219] btrfs_work_helper+0x27f/0x360 [btrfs]
>> [33812.505073] process_one_work+0x1b0/0x390
>> [33812.509101] worker_thread+0x3c/0x370
>> [33812.512779] ? process_one_work+0x390/0x390
>> [33812.516979] kthread+0xe3/0x110
>> [33812.520138] ? kthread_complete_and_exit+0x20/0x20
>> [33812.524945] ret_from_fork+0x1f/0x30
>> [33812.528542] </TASK>
>>
>> [33812.530751] task:umount state:D stack:0 pid:1465218 ppid:1464868 flags:0x00004002
>> [33812.539372] Call Trace:
>> [33812.541823] <TASK>
>> [33812.543938] __schedule+0x2cb/0x880
>> [33812.547441] ? wait_for_completion+0x83/0x160
>> [33812.551807] schedule+0x50/0xc0
>> [33812.554964] schedule_timeout+0x269/0x310
>> [33812.559005] ? wait_for_completion+0x83/0x160
>> [33812.563370] wait_for_completion+0xb5/0x160
>> [33812.567561] btrfs_wait_ordered_extents+0x346/0x450 [btrfs]
>> [33812.573197] btrfs_wait_ordered_roots+0x163/0x250 [btrfs]
>> [33812.578645] btrfs_sync_fs+0x3e/0x1c0 [btrfs]
>> [33812.583052] sync_filesystem+0x74/0x90
>> [33812.586830] generic_shutdown_super+0x22/0x120
>> [33812.591281] kill_anon_super+0x14/0x30
>> [33812.595049] btrfs_kill_super+0x12/0x20 [btrfs]
>> [33812.599629] deactivate_locked_super+0x2c/0x70
>> [33812.604079] cleanup_mnt+0xb8/0x140
>> [33812.607585] task_work_run+0x6a/0xb0
>> [33812.611168] exit_to_user_mode_prepare+0x1b9/0x1c0
>> [33812.615977] syscall_exit_to_user_mode+0x12/0x30
>> [33812.620617] do_syscall_64+0x67/0x80
>> [33812.624198] ? syscall_exit_to_user_mode+0x12/0x30
>> [33812.629007] ? do_syscall_64+0x67/0x80
>> [33812.632771] ? syscall_exit_to_user_mode+0x12/0x30
>> [33812.637579] ? do_syscall_64+0x67/0x80
>> [33812.641346] ? syscall_exit_to_user_mode+0x12/0x30
>> [33812.646144] ? do_syscall_64+0x67/0x80
>> [33812.649907] ? exc_page_fault+0x64/0x140
>> [33812.653847] entry_SYSCALL_64_after_hwframe+0x63/0xcd
>> [33812.658932] RIP: 0033:0x7f2eb454e56b
>> [33812.662510] RSP: 002b:00007ffdf683ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
>> [33812.670088] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f2eb454e56b
>> [33812.677242] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000563731e5b5a0
>> [33812.684394] RBP: 0000563731e5b370 R08: 0000000000000000 R09: 00007ffdf6839a00
>> [33812.691536] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>> [33812.698678] R13: 0000563731e5b5a0 R14: 0000563731e5b480 R15: 0000563731e5b370
>> [33812.705819] </TASK>
>>
>>
>> Best Regards
>> Wang Yugui (wangyugui@e16-tech.com)
>> 2023/03/07
>
> Thanks for the report. We have seen a similar regression on a similar
> test (generic/250) on most test machines except unfortunately mine.
>
> I have an idea for a fix, but it's hard to hack on it without a repro :(
>
> Hopefully should have a fix today or tomorrow.
>
> Boris
>
>>
>>> If an application is doing direct io to a btrfs file and experiences a
>>> page fault reading from the write buffer, iomap will issue a partial
>>> bio, and allow the fs to keep going. However, there was a subtle bug in
>>> this codepath in the btrfs dio iomap implementation that led to the
>>> partial write ending up as a gap in the file's extents and to be read
>>> back as zeros.
>>>
>>> The sequence of events in a partial write, lightly summarized and
>>> trimmed down for brevity is as follows:
>>>
>>> ====WRITING TASK====
>>> btrfs_direct_write
>>> __iomap_dio_write
>>> iomap_iter
>>> btrfs_dio_iomap_begin # create full ordered extent
>>> iomap_dio_bio_iter
>>> bio_iov_iter_get_pages # page fault; partial read
>>> submit_bio # partial bio
>>> iomap_iter
>>> btrfs_dio_iomap_end
>>> btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR;
>>> # submit to finish_ordered_fn wq
>>> fault_in_iov_iter_readable # btrfs_direct_write detects partial write
>>> __iomap_dio_write
>>> iomap_iter
>>> btrfs_dio_iomap_begin # create second partial ordered extent
>>> iomap_dio_bio_iter
>>> bio_iov_iter_get_pages # read all of remainder
>>> submit_bio # partial bio with all of remainder
>>> iomap_iter
>>> btrfs_dio_iomap_end # nothing exciting to do with ordered io
>>>
>>> ====DIO ENDIO====
>>> ==FIRST PARTIAL BIO==
>>> btrfs_dio_end_io
>>> btrfs_mark_ordered_io_finished # bytes_left > 0
>>> # don't submit to finish_ordered_fn wq
>>> ==SECOND PARTIAL BIO==
>>> btrfs_dio_end_io
>>> btrfs_mark_ordered_io_finished # bytes_left == 0
>>> # submit to finish_ordered_fn wq
>>>
>>> ====BTRFS FINISH ORDERED WQ====
>>> ==FIRST PARTIAL BIO==
>>> btrfs_finish_ordered_io # called by dio_iomap_end_io, sees
>>> # BTRFS_ORDERED_IOERR, just drops the
>>> # ordered_extent
>>> ==SECOND PARTIAL BIO==
>>> btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file
>>> # extents, csums, etc...
>>>
>>> The essence of the problem is that while btrfs_direct_write and iomap
>>> properly interact to submit all the correct bios, there is insufficient
>>> logic in the btrfs dio functions (btrfs_dio_iomap_begin,
>>> btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to
>>> ensure that every bio is at least a part of a completed ordered_extent.
>>> And it is completing an ordered_extent that results in crucial
>>> functionality like writing out a file extent for the range.
>>>
>>> More specifically, btrfs_dio_end_io treats the ordered extent as
>>> unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it.
>>> Thus, the finish io work doesn't result in file extents, csums, etc...
>>> In the aftermath, such a file behaves as though it has a hole in it,
>>> instead of the purportedly written data.
>>>
>>> We considered a few options for fixing the bug (apologies for any
>>> incorrect summary of a proposal which I didn't implement and fully
>>> understand):
>>> 1. treat the partial bio as if we had truncated the file, which would
>>> result in properly finishing it.
>>> 2. split the ordered extent when submitting a partial bio.
>>> 3. cache the ordered extent across calls to __iomap_dio_rw in
>>> iter->private, so that we could reuse it and correctly apply several
>>> bios to it.
>>>
>>> I had trouble with 1, and it felt the most like a hack, so I tried 2
>>> and 3. Since 3 has the benefit of also not creating an extra file
>>> extent, and avoids an ordered extent lookup during bio submission, it
>>> felt like the best option.
>>>
>>> A quick summary of the changes necessary to implement this cached
>>> ordered_extent behavior:
>>> - btrfs_direct_write keeps track of an ordered_extent for the duration
>>> of a call, possible across several __iomap_dio_rws.
>>> - zero the btrfs_dio_data before using it, since its fields constitute
>>> state now.
>>> - btrfs_dio_write uses dio_data to pass this ordered extent into and out
>>> of __iomap_dio_rw.
>>> - when the write is done, put the ordered_extent.
>>> - if the short write happens to be length 0, then we _don't_ get an
>>> extra bio, so we do need to cancel the ordered_extent like we used
>>> to (and ditch the cached ordered extent)
>>> - in btrfs_dio_iomap_begin, if the cached ordered extent is present,
>>> skip all the work of creating it, just look up the extent mapping and
>>> jump to setting up the iomap. (This part could likely be more
>>> elegant..)
>>>
>>> Thanks to Josef, Christoph, and Filipe with their help figuring out the
>>> bug and the fix.
>>>
>>> Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes")
>>> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947
>>> Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/
>>> Link: https://pastebin.com/3SDaH8C6
>>> Signed-off-by: Boris Burkov <boris@bur.io>
>>> ---
>>> fs/btrfs/btrfs_inode.h | 1 +
>>> fs/btrfs/file.c | 11 ++++++-
>>> fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++-----------
>>> 3 files changed, 67 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
>>> index 49a92aa65de1..87020aa58121 100644
>>> --- a/fs/btrfs/btrfs_inode.h
>>> +++ b/fs/btrfs/btrfs_inode.h
>>> @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from,
>>> ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter,
>>> size_t done_before);
>>> struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter,
>>> + struct btrfs_ordered_extent **ordered_extent,
>>> size_t done_before);
>>>
>>> extern const struct dentry_operations btrfs_dentry_operations;
>>> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
>>> index 5cc5a1faaef5..ec5c5355906b 100644
>>> --- a/fs/btrfs/file.c
>>> +++ b/fs/btrfs/file.c
>>> @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>>> ssize_t err;
>>> unsigned int ilock_flags = 0;
>>> struct iomap_dio *dio;
>>> + struct btrfs_ordered_extent *ordered_extent = NULL;
>>>
>>> if (iocb->ki_flags & IOCB_NOWAIT)
>>> ilock_flags |= BTRFS_ILOCK_TRY;
>>> @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>>> * got -EFAULT, faulting in the pages before the retry.
>>> */
>>> from->nofault = true;
>>> - dio = btrfs_dio_write(iocb, from, written);
>>> + dio = btrfs_dio_write(iocb, from, &ordered_extent, written);
>>> from->nofault = false;
>>>
>>> /*
>>> @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>>> goto relock;
>>> }
>>> }
>>> + /*
>>> + * We can't loop back to btrfs_dio_write, so we can drop the cached
>>> + * ordered extent. Typically btrfs_dio_iomap_end will run and put the
>>> + * ordered_extent, but this is needed to clean up in case of an error
>>> + * path breaking out of iomap_iter before the final iomap_end call.
>>> + */
>>> + if (ordered_extent)
>>> + btrfs_put_ordered_extent(ordered_extent);
>>>
>>> /*
>>> * If 'err' is -ENOTBLK or we have not written all data, then it means
>>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>>> index 44e9acc77a74..f1a59c5f3140 100644
>>> --- a/fs/btrfs/inode.c
>>> +++ b/fs/btrfs/inode.c
>>> @@ -81,6 +81,7 @@ struct btrfs_dio_data {
>>> struct extent_changeset *data_reserved;
>>> bool data_space_reserved;
>>> bool nocow_done;
>>> + struct btrfs_ordered_extent *ordered;
>>> };
>>>
>>> struct btrfs_dio_private {
>>> @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
>>> }
>>>
>>> static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
>>> + struct btrfs_dio_data *dio_data,
>>> const u64 start,
>>> const u64 len,
>>> const u64 orig_start,
>>> @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
>>> const int type)
>>> {
>>> struct extent_map *em = NULL;
>>> - int ret;
>>> + struct btrfs_ordered_extent *ordered;
>>>
>>> if (type != BTRFS_ORDERED_NOCOW) {
>>> em = create_io_em(inode, start, len, orig_start, block_start,
>>> @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
>>> if (IS_ERR(em))
>>> goto out;
>>> }
>>> - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start,
>>> - block_len, 0,
>>> - (1 << type) |
>>> - (1 << BTRFS_ORDERED_DIRECT),
>>> - BTRFS_COMPRESS_NONE);
>>> - if (ret) {
>>> + ordered = btrfs_alloc_ordered_extent(inode, start, len, len,
>>> + block_start, block_len, 0,
>>> + (1 << type) |
>>> + (1 << BTRFS_ORDERED_DIRECT),
>>> + BTRFS_COMPRESS_NONE);
>>> + if (IS_ERR(ordered)) {
>>> if (em) {
>>> free_extent_map(em);
>>> btrfs_drop_extent_map_range(inode, start,
>>> start + len - 1, false);
>>> }
>>> - em = ERR_PTR(ret);
>>> + em = ERR_PTR(PTR_ERR(ordered));
>>> + } else {
>>> + ASSERT(!dio_data->ordered);
>>> + dio_data->ordered = ordered;
>>> }
>>> out:
>>>
>>> @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
>>> }
>>>
>>> static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode,
>>> + struct btrfs_dio_data *dio_data,
>>> u64 start, u64 len)
>>> {
>>> struct btrfs_root *root = inode->root;
>>> @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode,
>>> if (ret)
>>> return ERR_PTR(ret);
>>>
>>> - em = btrfs_create_dio_extent(inode, start, ins.offset, start,
>>> + em = btrfs_create_dio_extent(inode, dio_data,
>>> + start, ins.offset, start,
>>> ins.objectid, ins.offset, ins.offset,
>>> ins.offset, BTRFS_ORDERED_REGULAR);
>>> btrfs_dec_block_group_reservations(fs_info, ins.objectid);
>>> @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
>>> }
>>> space_reserved = true;
>>>
>>> - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len,
>>> + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data,
>>> + start, len,
>>> orig_start, block_start,
>>> len, orig_block_len,
>>> ram_bytes, type);
>>> @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
>>> goto out;
>>> space_reserved = true;
>>>
>>> - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len);
>>> + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len);
>>> if (IS_ERR(em)) {
>>> ret = PTR_ERR(em);
>>> goto out;
>>> @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
>>> }
>>> }
>>>
>>> + if (dio_data->ordered) {
>>> + ASSERT(write);
>>> + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0,
>>> + dio_data->ordered->file_offset,
>>> + dio_data->ordered->bytes_left);
>>> + if (IS_ERR(em)) {
>>> + ret = PTR_ERR(em);
>>> + goto err;
>>> + }
>>> + goto map_iomap;
>>> + }
>>> memset(dio_data, 0, sizeof(*dio_data));
>>>
>>> /*
>>> @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
>>> else
>>> free_extent_state(cached_state);
>>>
>>> +map_iomap:
>>> /*
>>> * Translate extent map information to iomap.
>>> * We trim the extents (and move the addr) even though iomap code does
>>> @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length,
>>> if (submitted < length) {
>>> pos += submitted;
>>> length -= submitted;
>>> - if (write)
>>> - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL,
>>> - pos, length, false);
>>> - else
>>> + if (write) {
>>> + if (submitted == 0) {
>>> + btrfs_mark_ordered_io_finished(BTRFS_I(inode),
>>> + NULL, pos,
>>> + length, false);
>>> + btrfs_put_ordered_extent(dio_data->ordered);
>>> + dio_data->ordered = NULL;
>>> + }
>>> + } else {
>>> unlock_extent(&BTRFS_I(inode)->io_tree, pos,
>>> pos + length - 1, NULL);
>>> + }
>>> ret = -ENOTBLK;
>>> + } else {
>>> + /* On the last bio, release our cached ordered_extent */
>>> + if (write) {
>>> + btrfs_put_ordered_extent(dio_data->ordered);
>>> + dio_data->ordered = NULL;
>>> + }
>>> }
>>>
>>> if (write)
>>> @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = {
>>>
>>> ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before)
>>> {
>>> - struct btrfs_dio_data data;
>>> + struct btrfs_dio_data data = { };
>>>
>>> return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
>>> IOMAP_DIO_PARTIAL, &data, done_before);
>>> }
>>>
>>> struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter,
>>> + struct btrfs_ordered_extent **ordered_extent,
>>> size_t done_before)
>>> {
>>> - struct btrfs_dio_data data;
>>> + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent };
>>> + struct iomap_dio *dio;
>>>
>>> - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
>>> - IOMAP_DIO_PARTIAL, &data, done_before);
>>> + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
>>> + IOMAP_DIO_PARTIAL, &dio_data, done_before);
>>> + if (!IS_ERR_OR_NULL(dio))
>>> + *ordered_extent = dio_data.ordered;
>>> + return dio;
>>> }
>>>
>>> static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
>>> --
>>> 2.38.1
>>
>>
diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h index 49a92aa65de1..87020aa58121 100644 --- a/fs/btrfs/btrfs_inode.h +++ b/fs/btrfs/btrfs_inode.h @@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from, ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before); struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, + struct btrfs_ordered_extent **ordered_extent, size_t done_before); extern const struct dentry_operations btrfs_dentry_operations; diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 5cc5a1faaef5..ec5c5355906b 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) ssize_t err; unsigned int ilock_flags = 0; struct iomap_dio *dio; + struct btrfs_ordered_extent *ordered_extent = NULL; if (iocb->ki_flags & IOCB_NOWAIT) ilock_flags |= BTRFS_ILOCK_TRY; @@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) * got -EFAULT, faulting in the pages before the retry. */ from->nofault = true; - dio = btrfs_dio_write(iocb, from, written); + dio = btrfs_dio_write(iocb, from, &ordered_extent, written); from->nofault = false; /* @@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from) goto relock; } } + /* + * We can't loop back to btrfs_dio_write, so we can drop the cached + * ordered extent. Typically btrfs_dio_iomap_end will run and put the + * ordered_extent, but this is needed to clean up in case of an error + * path breaking out of iomap_iter before the final iomap_end call. + */ + if (ordered_extent) + btrfs_put_ordered_extent(ordered_extent); /* * If 'err' is -ENOTBLK or we have not written all data, then it means diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 44e9acc77a74..f1a59c5f3140 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -81,6 +81,7 @@ struct btrfs_dio_data { struct extent_changeset *data_reserved; bool data_space_reserved; bool nocow_done; + struct btrfs_ordered_extent *ordered; }; struct btrfs_dio_private { @@ -6976,6 +6977,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode, } static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, + struct btrfs_dio_data *dio_data, const u64 start, const u64 len, const u64 orig_start, @@ -6986,7 +6988,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, const int type) { struct extent_map *em = NULL; - int ret; + struct btrfs_ordered_extent *ordered; if (type != BTRFS_ORDERED_NOCOW) { em = create_io_em(inode, start, len, orig_start, block_start, @@ -6996,18 +6998,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, if (IS_ERR(em)) goto out; } - ret = btrfs_add_ordered_extent(inode, start, len, len, block_start, - block_len, 0, - (1 << type) | - (1 << BTRFS_ORDERED_DIRECT), - BTRFS_COMPRESS_NONE); - if (ret) { + ordered = btrfs_alloc_ordered_extent(inode, start, len, len, + block_start, block_len, 0, + (1 << type) | + (1 << BTRFS_ORDERED_DIRECT), + BTRFS_COMPRESS_NONE); + if (IS_ERR(ordered)) { if (em) { free_extent_map(em); btrfs_drop_extent_map_range(inode, start, start + len - 1, false); } - em = ERR_PTR(ret); + em = ERR_PTR(PTR_ERR(ordered)); + } else { + ASSERT(!dio_data->ordered); + dio_data->ordered = ordered; } out: @@ -7015,6 +7020,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode, } static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, + struct btrfs_dio_data *dio_data, u64 start, u64 len) { struct btrfs_root *root = inode->root; @@ -7030,7 +7036,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode, if (ret) return ERR_PTR(ret); - em = btrfs_create_dio_extent(inode, start, ins.offset, start, + em = btrfs_create_dio_extent(inode, dio_data, + start, ins.offset, start, ins.objectid, ins.offset, ins.offset, ins.offset, BTRFS_ORDERED_REGULAR); btrfs_dec_block_group_reservations(fs_info, ins.objectid); @@ -7375,7 +7382,8 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, } space_reserved = true; - em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len, + em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, + start, len, orig_start, block_start, len, orig_block_len, ram_bytes, type); @@ -7417,7 +7425,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map, goto out; space_reserved = true; - em = btrfs_new_extent_direct(BTRFS_I(inode), start, len); + em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len); if (IS_ERR(em)) { ret = PTR_ERR(em); goto out; @@ -7521,6 +7529,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, } } + if (dio_data->ordered) { + ASSERT(write); + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, + dio_data->ordered->file_offset, + dio_data->ordered->bytes_left); + if (IS_ERR(em)) { + ret = PTR_ERR(em); + goto err; + } + goto map_iomap; + } memset(dio_data, 0, sizeof(*dio_data)); /* @@ -7662,6 +7681,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start, else free_extent_state(cached_state); +map_iomap: /* * Translate extent map information to iomap. * We trim the extents (and move the addr) even though iomap code does @@ -7715,13 +7735,25 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length, if (submitted < length) { pos += submitted; length -= submitted; - if (write) - btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL, - pos, length, false); - else + if (write) { + if (submitted == 0) { + btrfs_mark_ordered_io_finished(BTRFS_I(inode), + NULL, pos, + length, false); + btrfs_put_ordered_extent(dio_data->ordered); + dio_data->ordered = NULL; + } + } else { unlock_extent(&BTRFS_I(inode)->io_tree, pos, pos + length - 1, NULL); + } ret = -ENOTBLK; + } else { + /* On the last bio, release our cached ordered_extent */ + if (write) { + btrfs_put_ordered_extent(dio_data->ordered); + dio_data->ordered = NULL; + } } if (write) @@ -7784,19 +7816,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = { ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before) { - struct btrfs_dio_data data; + struct btrfs_dio_data data = { }; return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, IOMAP_DIO_PARTIAL, &data, done_before); } struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter, + struct btrfs_ordered_extent **ordered_extent, size_t done_before) { - struct btrfs_dio_data data; + struct btrfs_dio_data dio_data = { .ordered = *ordered_extent }; + struct iomap_dio *dio; - return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, - IOMAP_DIO_PARTIAL, &data, done_before); + dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops, + IOMAP_DIO_PARTIAL, &dio_data, done_before); + if (!IS_ERR_OR_NULL(dio)) + *ordered_extent = dio_data.ordered; + return dio; } static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
If an application is doing direct io to a btrfs file and experiences a page fault reading from the write buffer, iomap will issue a partial bio, and allow the fs to keep going. However, there was a subtle bug in this codepath in the btrfs dio iomap implementation that led to the partial write ending up as a gap in the file's extents and to be read back as zeros. The sequence of events in a partial write, lightly summarized and trimmed down for brevity is as follows: ====WRITING TASK==== btrfs_direct_write __iomap_dio_write iomap_iter btrfs_dio_iomap_begin # create full ordered extent iomap_dio_bio_iter bio_iov_iter_get_pages # page fault; partial read submit_bio # partial bio iomap_iter btrfs_dio_iomap_end btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR; # submit to finish_ordered_fn wq fault_in_iov_iter_readable # btrfs_direct_write detects partial write __iomap_dio_write iomap_iter btrfs_dio_iomap_begin # create second partial ordered extent iomap_dio_bio_iter bio_iov_iter_get_pages # read all of remainder submit_bio # partial bio with all of remainder iomap_iter btrfs_dio_iomap_end # nothing exciting to do with ordered io ====DIO ENDIO==== ==FIRST PARTIAL BIO== btrfs_dio_end_io btrfs_mark_ordered_io_finished # bytes_left > 0 # don't submit to finish_ordered_fn wq ==SECOND PARTIAL BIO== btrfs_dio_end_io btrfs_mark_ordered_io_finished # bytes_left == 0 # submit to finish_ordered_fn wq ====BTRFS FINISH ORDERED WQ==== ==FIRST PARTIAL BIO== btrfs_finish_ordered_io # called by dio_iomap_end_io, sees # BTRFS_ORDERED_IOERR, just drops the # ordered_extent ==SECOND PARTIAL BIO== btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file # extents, csums, etc... The essence of the problem is that while btrfs_direct_write and iomap properly interact to submit all the correct bios, there is insufficient logic in the btrfs dio functions (btrfs_dio_iomap_begin, btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to ensure that every bio is at least a part of a completed ordered_extent. And it is completing an ordered_extent that results in crucial functionality like writing out a file extent for the range. More specifically, btrfs_dio_end_io treats the ordered extent as unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it. Thus, the finish io work doesn't result in file extents, csums, etc... In the aftermath, such a file behaves as though it has a hole in it, instead of the purportedly written data. We considered a few options for fixing the bug (apologies for any incorrect summary of a proposal which I didn't implement and fully understand): 1. treat the partial bio as if we had truncated the file, which would result in properly finishing it. 2. split the ordered extent when submitting a partial bio. 3. cache the ordered extent across calls to __iomap_dio_rw in iter->private, so that we could reuse it and correctly apply several bios to it. I had trouble with 1, and it felt the most like a hack, so I tried 2 and 3. Since 3 has the benefit of also not creating an extra file extent, and avoids an ordered extent lookup during bio submission, it felt like the best option. A quick summary of the changes necessary to implement this cached ordered_extent behavior: - btrfs_direct_write keeps track of an ordered_extent for the duration of a call, possible across several __iomap_dio_rws. - zero the btrfs_dio_data before using it, since its fields constitute state now. - btrfs_dio_write uses dio_data to pass this ordered extent into and out of __iomap_dio_rw. - when the write is done, put the ordered_extent. - if the short write happens to be length 0, then we _don't_ get an extra bio, so we do need to cancel the ordered_extent like we used to (and ditch the cached ordered extent) - in btrfs_dio_iomap_begin, if the cached ordered extent is present, skip all the work of creating it, just look up the extent mapping and jump to setting up the iomap. (This part could likely be more elegant..) Thanks to Josef, Christoph, and Filipe with their help figuring out the bug and the fix. Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes") Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947 Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/ Link: https://pastebin.com/3SDaH8C6 Signed-off-by: Boris Burkov <boris@bur.io> --- fs/btrfs/btrfs_inode.h | 1 + fs/btrfs/file.c | 11 ++++++- fs/btrfs/inode.c | 75 +++++++++++++++++++++++++++++++----------- 3 files changed, 67 insertions(+), 20 deletions(-)