@@ -7782,6 +7782,7 @@ static void btrfs_dio_submit_io(const struct iomap_iter *iter, struct bio *bio,
struct btrfs_dio_private *dip =
container_of(bbio, struct btrfs_dio_private, bbio);
struct btrfs_dio_data *dio_data = iter->private;
+ int err = 0;
btrfs_bio_init(bbio, BTRFS_I(iter->inode), btrfs_dio_end_io, bio->bi_private);
bbio->file_offset = file_offset;
@@ -7790,7 +7791,25 @@ static void btrfs_dio_submit_io(const struct iomap_iter *iter, struct bio *bio,
dip->bytes = bio->bi_iter.bi_size;
dio_data->submitted += bio->bi_iter.bi_size;
- btrfs_submit_bio(bbio, 0);
+ /*
+ * Check if we are doing a partial write. If we are, we need to split
+ * the ordered extent to match the submitted bio. Hang on to the
+ * remaining unfinishable ordered_extent in dio_data so that it can be
+ * cancelled in iomap_end to avoid a deadlock wherein faulting the
+ * remaining pages is blocked on the outstanding ordered extent.
+ */
+ if (iter->flags & IOMAP_WRITE) {
+ struct btrfs_ordered_extent *ordered = dio_data->ordered;
+
+ ASSERT(ordered);
+ if (bio->bi_iter.bi_size < ordered->num_bytes)
+ err = btrfs_extract_ordered_extent_bio(bbio, ordered, NULL,
+ &dio_data->ordered);
+ }
+ if (err)
+ btrfs_bio_end_io(bbio, err);
+ else
+ btrfs_submit_bio(bbio, 0);
}
static const struct iomap_ops btrfs_dio_iomap_ops = {
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. However, that turned out to re-introduce a deadlock which this code discarding the ordered_extent between faults was meant to fix in the first place. (Link to an explanation of the deadlock below) Therefore, go with fix #2, which requires a bit more setup work but fixes the corruption without introducing the deadlock, which is fundamentally caused by the ordered extent existing when we attempt to fault in a range that overlaps with it. Put succinctly, what this patch does is: when we submit a dio bio, check if it is partial against the ordered extent stored in dio_data, and if it is, extract the ordered_extent that matches the bio exactly out of the larger ordered_extent. Keep the remaining ordered_extent around in dio_data for cancellation in iomap_end. 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 Link: https://lore.kernel.org/linux-btrfs/20230315195231.GW10580@twin.jikos.cz/T/#t Signed-off-by: Boris Burkov <boris@bur.io> --- fs/btrfs/inode.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-)