From: Qu Wenruo <wqu@xxxxxxxx> [ Upstream commit a94d1d0cb3bf1983fcdf05b59d914dbff4f1f52c ] [BUG] The following script can cause unexpected fsync failure: #!/bin/bash dev=/dev/test/test mnt=/mnt/btrfs mkfs.btrfs -f $dev -b 512M > /dev/null mount $dev $mnt -o nospace_cache # Prealloc one extent xfs_io -f -c "falloc 8k 64m" $mnt/file1 # Fill the remaining data space xfs_io -f -c "pwrite 0 -b 4k 512M" $mnt/padding sync # Write into the prealloc extent xfs_io -c "pwrite 1m 16m" $mnt/file1 # Reflink then fsync, fsync would fail due to ENOSPC xfs_io -c "reflink $mnt/file1 8k 0 4k" -c "fsync" $mnt/file1 umount $dev The fsync fails with ENOSPC, and the last page of the buffered write is lost. [CAUSE] This is caused by: - Btrfs' back reference only has extent level granularity So write into shared extent must be COWed even only part of the extent is shared. So for above script we have: - fallocate Create a preallocated extent where we can do NOCOW write. - fill all the remaining data and unallocated space - buffered write into preallocated space As we have not enough space available for data and the extent is not shared (yet) we fall into NOCOW mode. - reflink Now part of the large preallocated extent is shared, later write into that extent must be COWed. - fsync triggers writeback But now the extent is shared and therefore we must fallback into COW mode, which fails with ENOSPC since there's not enough space to allocate data extents. [WORKAROUND] The workaround is to ensure any buffered write in the related extents (not just the reflink source range) get flushed before reflink/dedupe, so that NOCOW writes succeed that happened before reflinking succeed. The workaround is expensive, we could do it better by only flushing NOCOW range, but that needs extra accounting for NOCOW range. For now, fix the possible data loss first. Reviewed-by: Filipe Manana <fdmanana@xxxxxxxx> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> Signed-off-by: David Sterba <dsterba@xxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- fs/btrfs/ioctl.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index 2a1be0d1a698..5b4beebf138c 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -3999,6 +3999,27 @@ static int btrfs_remap_file_range_prep(struct file *file_in, loff_t pos_in, if (!same_inode) inode_dio_wait(inode_out); + /* + * Workaround to make sure NOCOW buffered write reach disk as NOCOW. + * + * Btrfs' back references do not have a block level granularity, they + * work at the whole extent level. + * NOCOW buffered write without data space reserved may not be able + * to fall back to CoW due to lack of data space, thus could cause + * data loss. + * + * Here we take a shortcut by flushing the whole inode, so that all + * nocow write should reach disk as nocow before we increase the + * reference of the extent. We could do better by only flushing NOCOW + * data, but that needs extra accounting. + * + * Also we don't need to check ASYNC_EXTENT, as async extent will be + * CoWed anyway, not affecting nocow part. + */ + ret = filemap_flush(inode_in->i_mapping); + if (ret < 0) + return ret; + ret = btrfs_wait_ordered_range(inode_in, ALIGN_DOWN(pos_in, bs), wb_len); if (ret < 0) -- 2.20.1
