FAILED: patch "[PATCH] btrfs: raid56: do not verify the content if there is no data checksum" failed to apply to 6.6-stable tree

FAILED: patch "[PATCH] btrfs: raid56: do not verify the content if there is no data checksum" failed to apply to 6.6-stable tree

[Sasha Levin]

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x d00b85308abee300dfb7e1ac642f38bab9ec7492
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107053346-failed-6.6-d00b85308abe@kernel.org' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From d00b85308abee300dfb7e1ac642f38bab9ec7492 Mon Sep 17 00:00:00 2001
From: Qu Wenruo <wqu@suse.com>
Date: Wed, 30 Sep 2026 18:29:58 +0930
Subject: [PATCH] btrfs: raid56: do not verify the content if there is no data
 checksum

[BUG]
There is a bug report that fio can fail to sync the data on a degraded
RAID56 mount:

  # mkfs.btrfs -m raid1 -d raid5 -f $dev1 $dev2 $dev3
  # mount $dev1 $mnt
  # btrfs balance start -d $mnt
  # umount $mnt
  # btrfs dev scan -u $dev2
  # wipefs -fa $dev2
  # mount -o degraded $dev1 $mnt
  # fio --name=degraded-write --directory=$mnt --rw=randwrite --fdatasync=4 \
		--bs=4k --size=512M --randrepeat=1

The run will fail at fsync:

  fio: io_u error on file /mnt/btrfs/degraded-write.0.0: Input/output error: datasync offset=142254080, buflen=0
  fio: pid=2064, err=5/file:io_u.c:2008, func=io_u error, error=Input/output error

[CAUSE]
With extra debug output, it shows the problem pretty well:

  rbio=2876243968 faila=0 sectornr=0
  rbio=2876243968 error=0-15 csum=2 bio=15

The first line shows the rbio's full stripe logical, and stripe/sector
number when verify_one_sector() failed.

The second line shows more detail about the rbio, the sector range [0, 16)
is corrupted, which indicates the first data stripe is corrupted, matching
the missing device.

The important part is that the csum bitmap shows the data csum is only
for sector number 2, meanwhile the failed verification is at sector
number 0.

This means we did the checksum verification even if no corresponding
checksum is populated.

[FIX]
There is already a similar check at verify_bio_data_sectors(), but
unfortunately there is no such check at verify_one_sector().

Add the missing check to skip the csum verification if there is no
corresponding data checksum.

Reported-by: Bartosz Fenski <bartosz@fenski.pl>
Link: https://lore.kernel.org/linux-btrfs/a72409af-45ef-45df-9970-2e38bcb22ea6@fenski.pl/
Fixes: 7a3150723061 ("btrfs: raid56: do data csum verification during RMW cycle")
CC: stable@vger.kernel.org
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/raid56.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
index 8ec24dbb180f9..5ec31299f1c30 100644
--- a/fs/btrfs/raid56.c
+++ b/fs/btrfs/raid56.c
@@ -1889,6 +1889,8 @@ static int verify_one_sector(struct btrfs_raid_bio *rbio,
 {
 	struct btrfs_fs_info *fs_info = rbio->bioc->fs_info;
 	phys_addr_t *paddrs;
+	const unsigned int total_sector_nr =
+		stripe_nr * rbio->stripe_nsectors + sector_nr;
 	u8 csum_buf[BTRFS_CSUM_SIZE];
 	u8 *csum_expected;
 
@@ -1898,6 +1900,11 @@ static int verify_one_sector(struct btrfs_raid_bio *rbio,
 	/* No way to verify P/Q as they are not covered by data csum. */
 	if (stripe_nr >= rbio->nr_data)
 		return 0;
+
+	/* No data checksum for this sector. */
+	if (!test_bit(total_sector_nr, rbio->csum_bitmap))
+		return 0;
+
 	/*
 	 * If we're rebuilding a read, we have to use pages from the
 	 * bio list if possible.
@@ -1908,9 +1915,7 @@ static int verify_one_sector(struct btrfs_raid_bio *rbio,
 		paddrs = rbio_stripe_paddrs(rbio, stripe_nr, sector_nr);
 	}
 
-	csum_expected = rbio->csum_buf +
-			(stripe_nr * rbio->stripe_nsectors + sector_nr) *
-			fs_info->csum_size;
+	csum_expected = rbio->csum_buf + total_sector_nr * fs_info->csum_size;
 	calculate_block_csum_paddrs(fs_info, paddrs, csum_buf);
 	if (unlikely(memcmp(csum_buf, csum_expected, fs_info->csum_size) != 0))
 		return -EIO;
-- 
2.53.0