[PATCH 6.7 089/162] btrfs: send: dont issue unnecessary zero writes for trailing hole

Greg Kroah-Hartman
2024-03-04T21:30:06Z

From: Filipe Manana <fdmanana@suse.com>

commit 5897710b28cabab04ea6c7547f27b7989de646ae upstream.

If we have a sparse file with a trailing hole (from the last extent's end
to i_size) and then create an extent in the file that ends before the
file's i_size, then when doing an incremental send we will issue a write
full of zeroes for the range that starts immediately after the new extent
ends up to i_size. While this isn't incorrect because the file ends up
with exactly the same data, it unnecessarily results in using extra space
at the destination with one or more extents full of zeroes instead of
having a hole. In same cases this results in using megabytes or even
gigabytes of unnecessary space.

Example, reproducer:

$ cat test.sh
#!/bin/bash

DEV=/dev/sdh
MNT=/mnt/sdh

mkfs.btrfs -f $DEV
mount $DEV $MNT

# Create 1G sparse file.
xfs_io -f -c "truncate 1G" $MNT/foobar

# Create base snapshot.
btrfs subvolume snapshot -r $MNT $MNT/mysnap1

# Create send stream (full send) for the base snapshot.
btrfs send -f /tmp/1.snap $MNT/mysnap1

# Now write one extent at the beginning of the file and one somewhere
# in the middle, leaving a gap between the end of this second extent
# and the file's size.
xfs_io -c "pwrite -S 0xab 0 128K" \
-c "pwrite -S 0xcd 512M 128K" \
$MNT/foobar

# Now create a second snapshot which is going to be used for an
# incremental send operation.
btrfs subvolume snapshot -r $MNT $MNT/mysnap2

# Create send stream (incremental send) for the second snapshot.
btrfs send -p $MNT/mysnap1 -f /tmp/2.snap $MNT/mysnap2

# Now recreate the filesystem by receiving both send streams and
# verify we get the same content that the original filesystem had
# and file foobar has only two extents with a size of 128K each.
umount $MNT
mkfs.btrfs -f $DEV
mount $DEV $MNT

btrfs receive -f /tmp/1.snap $MNT
btrfs receive -f /tmp/2.snap $MNT

echo -e "\nFile fiemap in the second snapshot:"
# Should have:
#
# 128K extent at file range [0, 128K[
# hole at file range [128K, 512M[
# 128K extent file range [512M, 512M + 128K[
# hole at file range [512M + 128K, 1G[
xfs_io -r -c "fiemap -v" $MNT/mysnap2/foobar

# File should be using 256K of data (two 128K extents).
echo -e "\nSpace used by the file: $(du -h $MNT/mysnap2/foobar | cut -f 1)"

umount $MNT

Running the test, we can see with fiemap that we get an extent for the
range [512M, 1G[, while in the source filesystem we have an extent for
the range [512M, 512M + 128K[ and a hole for the rest of the file (the
range [512M + 128K, 1G[):

$ ./test.sh
(...)
File fiemap in the second snapshot:
/mnt/sdh/mysnap2/foobar:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..255]: 26624..26879 256 0x0
1: [256..1048575]: hole 1048320
2: [1048576..2097151]: 2156544..3205119 1048576 0x1

Space used by the file: 513M

This happens because once we finish processing an inode, at
finish_inode_if_needed(), we always issue a hole (write operations full
of zeros) if there's a gap between the end of the last processed extent
and the file's size, even if that range is already a hole in the parent
snapshot. Fix this by issuing the hole only if the range is not already
a hole.

After this change, running the test above, we get the expected layout:

$ ./test.sh
(...)
File fiemap in the second snapshot:
/mnt/sdh/mysnap2/foobar:
EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS
0: [0..255]: 26624..26879 256 0x0
1: [256..1048575]: hole 1048320
2: [1048576..1048831]: 26880..27135 256 0x1
3: [1048832..2097151]: hole 1048320

Space used by the file: 256K

A test case for fstests will follow soon.

CC: stable@vger.kernel.org # 6.1+
Reported-by: Dorai Ashok S A <dash.btrfs@inix.me>
Link: https://lore.kernel.org/linux-btrfs/c0bf7818-9c45-46a8-b3d3-513230d0c86e@inix.me/
Reviewed-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/send.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -6705,11 +6705,20 @@ static int finish_inode_if_needed(struct
if (ret)
goto out;
}
- if (sctx->cur_inode_last_extent <
- sctx->cur_inode_size) {
- ret = send_hole(sctx, sctx->cur_inode_size);
- if (ret)
+ if (sctx->cur_inode_last_extent < sctx->cur_inode_size) {
+ ret = range_is_hole_in_parent(sctx,
+ sctx->cur_inode_last_extent,
+ sctx->cur_inode_size);
+ if (ret < 0) {
goto out;
+ } else if (ret == 0) {
+ ret = send_hole(sctx, sctx->cur_inode_size);
+ if (ret < 0)
+ goto out;
+ } else {
+ /* Range is already a hole, skip. */
+ ret = 0;
+ }
}
}
if (need_truncate) {