[PATCH v2] btrfs: raid56: fix scrub read assembly submitting no reads

[Mykola Lysenko <nickolay.lysenko@gmail.com>]

Commit 5387bd958180 ("btrfs: raid56: remove sector_ptr structure")
converted the bio-list membership checks from sector pointers to
physical addresses. The two conversions in rmw_assemble_write_bios()
kept their polarity (skip the sector when it is NOT in the bio list,
i.e. when there is nothing to write), but scrub_assemble_read_bios()
has the opposite polarity -- skip the sector when it IS in the bio
list, because then there is nothing to read -- and the conversion
flipped it:

	-	sector = sector_in_rbio(rbio, stripe, sectornr, 1);
	-	if (sector)
	+	paddr = sector_paddr_in_rbio(rbio, stripe, sectornr, 1);
	+	if (paddr == INVALID_PADDR)
		continue;

Since a parity-scrub rbio's bio list only holds the empty completion
bio, the result is that scrub_assemble_read_bios() submits no reads at
all. finish_parity_scrub() then compares the parity it computes from
the (cached, correct) data stripes against whatever happens to be in
the freshly allocated, uninitialized stripe pages:

  - if the garbage differs from the computed parity, the sector is
    "repaired" and written back -- accidentally producing the correct
    on-disk result;
  - if a recycled page happens to still hold the old (correct) parity
    content, the sector is deemed clean, dropped from dbitmap, and the
    actually-corrupt on-disk parity is left in place. (Scrub reports
    no errors either way: there is no counter for P/Q corruption by
    design, so the bug here is purely the failure to read and repair.)

The second case is intermittent because it depends on page-allocator
recycling. Observed with fstests btrfs/297 (raid5, 2 devices): the
corrupted P stripe intermittently stays corrupt after a scrub --
roughly 1/10 runs on x86-64 KVM and up to 7/8 on a UML build whose
timing favors page reuse.

Since the bio-list check can never be true for a parity-scrub rbio --
raid56_parity_alloc_scrub_rbio() adds a single empty completion bio
(asserting bi_size == 0), bio_paddrs[] is only populated by
index_rbio_pages() which is never called for BTRFS_RBIO_PARITY_SCRUB,
and rbio_can_merge() refuses to merge rbios of different operations --
remove the dead check entirely and assert the invariant instead, as
suggested by Qu Wenruo.

After this fix the injected corruption is read, detected and repaired
in every run (8/8 UML, 10/10 KVM), and the new assertion never fires
across the full fstests raid group.

Fixes: 5387bd958180 ("btrfs: raid56: remove sector_ptr structure")
CC: stable@vger.kernel.org # 7.1+
Suggested-by: Qu Wenruo <quwenruo.btrfs@gmx.com>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Mykola Lysenko <nickolay.lysenko@gmail.com>
---
v2: per Qu Wenruo's review -- the membership check is dead code in
either polarity for a scrub rbio, so remove it and ASSERT the
invariant instead of restoring the original polarity; clarify in the
message that the absence of P/Q error reporting is by design; move the
AI-usage disclosure to an Assisted-by tag per
Documentation/process/coding-assistants.rst.

v1: https://lore.kernel.org/linux-btrfs/20260716174511.8738-1-nickolay.lysenko@gmail.com/

Note: UML (User-Mode Linux) results above come from
https://github.com/mykola-lysenko/btrfs-uml-fstests/ -- for reference
only.

 fs/btrfs/raid56.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

--- a/fs/btrfs/raid56.c
+++ b/fs/btrfs/raid56.c
@@ -2909,13 +2909,12 @@ static int scrub_assemble_read_bios(struct btrfs_raid_bio *rbio)
 			continue;

 		/*
-		 * We want to find all the sectors missing from the rbio and
-		 * read them from the disk. If sector_paddr_in_rbio() finds a sector
-		 * in the bio list we don't need to read it off the stripe.
+		 * A parity-scrub rbio carries no data in its bio list: the
+		 * only bio there is the empty completion bio added by
+		 * raid56_parity_alloc_scrub_rbio().  Every sector is read
+		 * from the stripe, so only assert that invariant here.
 		 */
-		paddrs = sector_paddrs_in_rbio(rbio, stripe, sectornr, 1);
-		if (paddrs == NULL)
-			continue;
+		ASSERT(!sector_paddrs_in_rbio(rbio, stripe, sectornr, 1));

 		paddrs = rbio_stripe_paddrs(rbio, stripe, sectornr);
 		/*
--
2.43.0