[PATCH v2 1/5] btrfs: don't drop the error of the last split bio

[Josef Bacik <josef@toxicpanda.com>]

When a btrfs_bio had to be split, btrfs_bio_end_io() saves the first error
of any of its parts in the status field of the original btrfs_bio. The
part that completes last is supposed to load it into bi_status of the
original bio before calling the end_io handler, but that is only done if
that last part was successful:

	if (atomic_dec_and_test(&bbio->pending_ios)) {
		/* Load split bio's error which might be set above. */
		if (status == BLK_STS_OK)
			bbio->bio.bi_status = READ_ONCE(bbio->status);

If the last part failed, the assumption is that bi_status was set at the
top of the function. That is true if the last part is what is left of the
original bio. If it is a split bio, the assignment was done to the split
bio, which was freed right after. If the original bio has completed before
without an error, its bi_status is BLK_STS_OK and stays like that:

  original bio (tail)                 split bio (head)

  btrfs_bio_end_io(orig, OK)
    orig->bio.bi_status = OK
    pending_ios 2 -> 1
                                      btrfs_bio_end_io(split, IOERR)
                                        split->bio.bi_status = IOERR
                                        bio_put(split)
                                        cmpxchg(&orig->status, OK, IOERR)
                                        pending_ios 1 -> 0
                                        orig->end_io(), bi_status is OK

The end_io handler covers the whole range, so for a buffered read all the
folios are marked uptodate, including the ones that were never filled, and
read(2) returns their content instead of -EIO. For a write the ordered
extent is completed as if all the data had reached the device.

This can be reproduced with raid0 data on two devices, one of them slower
than the other and with a bad block, by reading a file whose last stripe
is on the fast device.

The saved status has the first error of all the parts, or BLK_STS_OK if
there was none, so always use it.

Fixes: d48e1dea3931 ("btrfs: fix error propagation of split bios")
CC: stable@vger.kernel.org # 6.12+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Tested-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/bio.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
index 2c6234f6182f..fc0b021cdefb 100644
--- a/fs/btrfs/bio.c
+++ b/fs/btrfs/bio.c
@@ -127,9 +127,12 @@ void btrfs_bio_end_io(struct btrfs_bio *bbio, blk_status_t status)
 		cmpxchg(&bbio->status, BLK_STS_OK, status);
 
 	if (atomic_dec_and_test(&bbio->pending_ios)) {
-		/* Load split bio's error which might be set above. */
-		if (status == BLK_STS_OK)
-			bbio->bio.bi_status = READ_ONCE(bbio->status);
+		/*
+		 * Load the first error, if any. It may come from a split bio,
+		 * including the one we were called for, in which case nothing
+		 * stored it in the original bio yet.
+		 */
+		bbio->bio.bi_status = READ_ONCE(bbio->status);
 
 		if (bbio_has_ordered_extent(bbio)) {
 			struct btrfs_ordered_extent *ordered = bbio->ordered;

-- 
2.55.0