[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 two reasons:
- Btrfs' back reference only has extent level granularity
So write into shared extent must be CoWed even only part of the extent
is shared.
- Btrfs doesn't reserve data space for NOCOW buffered write if low on
data space
So for above script we have:
- fallocate
Create a preallocated extent where we can do NOCOW write.
- padding buffered write to use up all data space
- buffered write into preallocated space
As we have no data space remaining, we have to do NOCOW check and
reserve no data space.
- reflink
Now part of the large preallocated extent is shared, later write
into that extent must be CoWed.
- writeback kicks in for fsync
buffered write into that preallocated space must be CoWed.
However we have no data space left at all, we fail
btrfs_run_delalloc_range() with ENOSPC, causing fsync failure.
[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 NOCOW write could reach disk as NOCOW before we increase the reference.
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.
Signed-off-by: Qu Wenruo <w...@suse.com>
---
changelog:
RFC->v1:
- Use better words for commit message and comment.
- Move the whole inode flushing to btrfs_remap_file_range_prep().
This also covers dedupe.
- Update the reproducer to fail explicitly.
- Remove false statement on transaction abort.
---
fs/btrfs/ioctl.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 6dafa857bbb9..87a0ec0591cd 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -4001,8 +4001,21 @@ static int btrfs_remap_file_range_prep(struct file
*file_in, loff_t pos_in,
if (!same_inode)
inode_dio_wait(inode_out);
- ret = btrfs_wait_ordered_range(inode_in, ALIGN_DOWN(pos_in, bs),
- wb_len);
+ /*
+ * 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.
+ */
+ ret = btrfs_wait_ordered_range(inode_in, 0, (u64)-1);
if (ret < 0)
return ret;
ret = btrfs_wait_ordered_range(inode_out, ALIGN_DOWN(pos_out, bs),
--
2.21.0
