| Message ID | 20200615174913.14943-1-fdmanana@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/4] Btrfs: fix hang on snapshot creation after RWF_NOWAIT write | expand |
On 6/15/20 1:49 PM, fdmanana@kernel.org wrote: > From: Filipe Manana <fdmanana@suse.com> > > If we attempt to do a RWF_NOWAIT write against a file range for which we > can only do NOCOW for a part of it, due to the existence of holes or > shared extents for example, we proceed with the write as if it were > possible to NOCOW the whole range. > > Example: > > $ mkfs.btrfs -f /dev/sdb > $ mount /dev/sdb /mnt > > $ touch /mnt/sdj/bar > $ chattr +C /mnt/sdj/bar > > $ xfs_io -d -c "pwrite -S 0xab -b 256K 0 256K" /mnt/bar > wrote 262144/262144 bytes at offset 0 > 256 KiB, 1 ops; 0.0003 sec (694.444 MiB/sec and 2777.7778 ops/sec) > > $ xfs_io -c "fpunch 64K 64K" /mnt/bar > $ sync > > $ xfs_io -d -c "pwrite -N -V 1 -b 128K -S 0xfe 0 128K" /mnt/bar > wrote 131072/131072 bytes at offset 0 > 128 KiB, 1 ops; 0.0007 sec (160.051 MiB/sec and 1280.4097 ops/sec) > > This last write should fail with -EAGAIN since the file range from 64K to > 128Kb is a hole. On xfs it fails, as expected, but on ext4 it currently > succeeds because apparently it is expensive to check if there are extents > allocated for the whole range, but I'll check with the ext4 people. > > Fix the issue by checking if check_can_nocow() returns a number of > NOCOW'able bytes smaller then the requested number of bytes, and if it > does return -EAGAIN. > > Fixes: edf064e7c6fec3 ("btrfs: nowait aio support") > Signed-off-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: Josef Bacik <josef@toxicpanda.com> Thanks, Josef
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 04faa04fccd1..78481d1e5e6e 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -1904,18 +1904,28 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb, pos = iocb->ki_pos; count = iov_iter_count(from); if (iocb->ki_flags & IOCB_NOWAIT) { + size_t nocow_bytes = count; /* * We will allocate space in case nodatacow is not set, * so bail */ if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW | BTRFS_INODE_PREALLOC)) || - check_can_nocow(BTRFS_I(inode), pos, &count) <= 0) { + check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes) <= 0) { inode_unlock(inode); return -EAGAIN; } /* check_can_nocow() locks the snapshot lock on success */ btrfs_drew_write_unlock(&root->snapshot_lock); + /* + * There are holes in the range or parts of the range that must + * be COWed (shared extents, RO block groups, etc), so just bail + * out. + */ + if (nocow_bytes < count) { + inode_unlock(inode); + return -EAGAIN; + } } current->backing_dev_info = inode_to_bdi(inode);