block: don't allow splitting of a REQ_NOWAIT bio

commit 9cea62b2cbabff8ed46f2df17778b624ad9dd25a upstream.

If we split a bio marked with REQ_NOWAIT, then we can trigger spurious
EAGAIN if constituent parts of that split bio end up failing request
allocations. Parts will complete just fine, but just a single failure
in one of the chained bios will yield an EAGAIN final result for the
parent bio.

Return EAGAIN early if we end up needing to split such a bio, which
allows for saner recovery handling.

Cc: stable@vger.kernel.org # 5.15+
Link: https://github.com/axboe/liburing/issues/766
Reported-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index bbe66a9..bb26db9 100644 (file)
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -279,6 +279,16 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
        *segs = nsegs;
        return NULL;
 split:
+       /*
+        * We can't sanely support splitting for a REQ_NOWAIT bio. End it
+        * with EAGAIN if splitting is required and return an error pointer.
+        */
+       if (bio->bi_opf & REQ_NOWAIT) {
+               bio->bi_status = BLK_STS_AGAIN;
+               bio_endio(bio);
+               return ERR_PTR(-EAGAIN);
+       }
+
        *segs = nsegs;
 
        /*