[PATCH 5.19 100/158] btrfs: update generation of hole file extent item when merging holes

Greg Kroah-Hartman
2022-08-29T11:49:33Z

From: Filipe Manana <fdmanana@suse.com>

commit e6e3dec6c3c288d556b991a85d5d8e3ee71e9046 upstream.

When punching a hole into a file range that is adjacent with a hole and we
are not using the no-holes feature, we expand the range of the adjacent
file extent item that represents a hole, to save metadata space.

However we don't update the generation of hole file extent item, which
means a full fsync will not log that file extent item if the fsync happens
in a later transaction (since commit 7f30c07288bb9e ("btrfs: stop copying
old file extents when doing a full fsync")).

For example, if we do this:

$ mkfs.btrfs -f -O ^no-holes /dev/sdb
$ mount /dev/sdb /mnt
$ xfs_io -f -c "pwrite -S 0xab 2M 2M" /mnt/foobar
$ sync

We end up with 2 file extent items in our file:

1) One that represents the hole for the file range [0, 2M), with a
generation of 7;

2) Another one that represents an extent covering the range [2M, 4M).

After that if we do the following:

$ xfs_io -c "fpunch 2M 2M" /mnt/foobar

We end up with a single file extent item in the file, which represents a
hole for the range [0, 4M) and with a generation of 7 - because we end
dropping the data extent for range [2M, 4M) and then update the file
extent item that represented the hole at [0, 2M), by increasing
length from 2M to 4M.

Then doing a full fsync and power failing:

$ xfs_io -c "fsync" /mnt/foobar
<power failure>

will result in the full fsync not logging the file extent item that
represents the hole for the range [0, 4M), because its generation is 7,
which is lower than the generation of the current transaction (8).
As a consequence, after mounting again the filesystem (after log replay),
the region [2M, 4M) does not have a hole, it still points to the
previous data extent.

So fix this by always updating the generation of existing file extent
items representing holes when we merge/expand them. This solves the
problem and it's the same approach as when we merge prealloc extents that
got written (at btrfs_mark_extent_written()). Setting the generation to
the current transaction's generation is also what we do when merging
the new hole extent map with the previous one or the next one.

A test case for fstests, covering both cases of hole file extent item
merging (to the left and to the right), will be sent soon.

Fixes: 7f30c07288bb9e ("btrfs: stop copying old file extents when doing a full fsync")
CC: stable@vger.kernel.org # 5.18+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/file.c | 2 ++
1 file changed, 2 insertions(+)

--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2483,6 +2483,7 @@ static int fill_holes(struct btrfs_trans
btrfs_set_file_extent_num_bytes(leaf, fi, num_bytes);
btrfs_set_file_extent_ram_bytes(leaf, fi, num_bytes);
btrfs_set_file_extent_offset(leaf, fi, 0);
+ btrfs_set_file_extent_generation(leaf, fi, trans->transid);
btrfs_mark_buffer_dirty(leaf);
goto out;
}
@@ -2499,6 +2500,7 @@ static int fill_holes(struct btrfs_trans
btrfs_set_file_extent_num_bytes(leaf, fi, num_bytes);
btrfs_set_file_extent_ram_bytes(leaf, fi, num_bytes);
btrfs_set_file_extent_offset(leaf, fi, 0);
+ btrfs_set_file_extent_generation(leaf, fi, trans->transid);
btrfs_mark_buffer_dirty(leaf);
goto out;
}