[PATCH 2/2] block: don't allow splitting of a REQ_NOWAIT bio

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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@xxxxxxxxxxxxxxx # 5.15+
Link: https://github.com/axboe/liburing/issues/766
Reported-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
 block/blk-merge.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 071c5f8cf0cf..b7c193d67185 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -309,6 +309,16 @@ static struct bio *bio_split_rw(struct bio *bio, const struct queue_limits *lim,
 	*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;
 
 	/*
-- 
2.39.0




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