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Date:   Thu, 13 Apr 2017 10:06:29 +0200
From:   Johannes Thumshirn <>
To:     Jens Axboe <>, Omar Sandoval <>
Cc:     Ming Lei <>,
        Bart Van Assche <>,
        Hannes Reinecke <>, Christoph Hellwig <>,
        Linux Block Layer Mailinglist <>,
        Linux Kernel Mailinglist <>,
        Johannes Thumshirn <>
Subject: [PATCH] block: bios with an offset are always gappy

Doing a mkfs.btrfs on a (qemu emulated) PCIe NVMe causes a kernel panic
in nvme_setup_prps() because the dma_len will drop below zero but the
length not.

A git bisect tracked the behaviour down to commit 729204ef49ec ("block:
relax check on sg gap"). Since commit 729204ef49ec a bio's offsets are not
taken into account in the decision if the bio will gap any more. Restore
the old behavior of checking bio offsets as well for the decision if a
bio will gap.

Signed-off-by: Johannes Thumshirn <>
Fixes: 729204ef49ec ("block: relax check on sg gap")
Cc: Ming Lei <>
 include/linux/blkdev.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 7548f332121a..a03b7196209e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1677,11 +1677,14 @@ static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
 	if (bio_has_data(prev) && queue_virt_boundary(q)) {
 		struct bio_vec pb, nb;
+		bool offset;
 		bio_get_last_bvec(prev, &pb);
 		bio_get_first_bvec(next, &nb);
-		if (!bios_segs_mergeable(q, prev, &pb, &nb))
+		offset = pb.bv_offset || nb.bv_offset;
+		if (offset || !bios_segs_mergeable(q, prev, &pb, &nb))
 			return __bvec_gap_to_prev(q, &pb, nb.bv_offset);

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