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From: Christoph Hellwig <hch@lst.de>
To: axboe@fb.com
Cc: ming.lei@redhat.com, linux-block@vger.kernel.org
Subject: [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments
Date: Thu, 16 May 2019 10:40:55 +0200
Message-ID: <20190516084058.20678-2-hch@lst.de> (raw)
In-Reply-To: <20190516084058.20678-1-hch@lst.de>

Currently ll_merge_requests_fn, unlike all other merge functions,
reduces nr_phys_segments by one if the last segment of the previous,
and the first segment of the next segement are contigous.  While this
seems like a nice solution to avoid building smaller than possible
requests it causes a mismatch between the segments actually present
in the request and those iterated over by the bvec iterators, including
__rq_for_each_bio.  This could cause overwrites of too small kmalloc
allocations in any driver using ranged discard, or also mistrigger
the single segment optimization in the nvme-pci driver.

We could possibly work around this by making the bvec iterators take
the front and back segment size into account, but that would require
moving them from the bio to the bio_iter and spreading this mess
over all users of bvecs.  Or we could simply remove this optimization
under the assumption that most users already build good enough bvecs,
and that the bio merge patch never cared about this optimization
either.  The latter is what this patch does.

Fixes: b35ba01ea697 ("nvme: support ranged discard requests")
Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
Fixes: 297910571f08 ("nvme-pci: optimize mapping single segment requests using SGLs")
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 block/blk-merge.c | 23 +----------------------
 1 file changed, 1 insertion(+), 22 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 21e87a714a73..80a5a0facb87 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -358,7 +358,6 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 	unsigned front_seg_size;
 	struct bio *fbio, *bbio;
 	struct bvec_iter iter;
-	bool new_bio = false;
 
 	if (!bio)
 		return 0;
@@ -379,31 +378,12 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 	nr_phys_segs = 0;
 	for_each_bio(bio) {
 		bio_for_each_bvec(bv, bio, iter) {
-			if (new_bio) {
-				if (seg_size + bv.bv_len
-				    > queue_max_segment_size(q))
-					goto new_segment;
-				if (!biovec_phys_mergeable(q, &bvprv, &bv))
-					goto new_segment;
-
-				seg_size += bv.bv_len;
-
-				if (nr_phys_segs == 1 && seg_size >
-						front_seg_size)
-					front_seg_size = seg_size;
-
-				continue;
-			}
-new_segment:
 			bvec_split_segs(q, &bv, &nr_phys_segs, &seg_size,
 					&front_seg_size, NULL, UINT_MAX);
-			new_bio = false;
 		}
 		bbio = bio;
-		if (likely(bio->bi_iter.bi_size)) {
+		if (likely(bio->bi_iter.bi_size))
 			bvprv = bv;
-			new_bio = true;
-		}
 	}
 
 	fbio->bi_seg_front_size = front_seg_size;
@@ -725,7 +705,6 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 			req->bio->bi_seg_front_size = seg_size;
 		if (next->nr_phys_segments == 1)
 			next->biotail->bi_seg_back_size = seg_size;
-		total_phys_segments--;
 	}
 
 	if (total_phys_segments > queue_max_segments(q))
-- 
2.20.1


  reply index

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16  8:40 fix nr_phys_segments vs iterators accounting v2 Christoph Hellwig
2019-05-16  8:40 ` Christoph Hellwig [this message]
2019-05-16  8:48   ` [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments Hannes Reinecke
2019-05-16 13:17   ` Ming Lei
2019-05-17 23:02     ` Ming Lei
2019-05-20 11:11     ` Christoph Hellwig
2019-05-21  1:04       ` Ming Lei
2019-05-16  8:40 ` [PATCH 2/4] block: force an unlimited segment size on queues with a virt boundary Christoph Hellwig
2019-05-16  8:49   ` Hannes Reinecke
2019-05-16  8:40 ` [PATCH 3/4] block: remove the segment size check in bio_will_gap Christoph Hellwig
2019-05-16  8:49   ` Hannes Reinecke
2019-05-16  8:40 ` [PATCH 4/4] block: remove the bi_seg_{front,back}_size fields in struct bio Christoph Hellwig
2019-05-16  8:50   ` Hannes Reinecke
2019-05-20 11:17 ` fix nr_phys_segments vs iterators accounting v2 Christoph Hellwig
2019-05-21  1:09   ` Jens Axboe
2019-05-21  1:17     ` Ming Lei
2019-05-21  1:20       ` Jens Axboe
2019-05-21  1:29         ` Ming Lei
2019-05-21  5:11           ` Christoph Hellwig
2019-05-21  7:01 fix nr_phys_segments vs iterators accounting v3 Christoph Hellwig
2019-05-21  7:01 ` [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments Christoph Hellwig
2019-05-21  8:05   ` Ming Lei

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