block: don’t decrement nr_phys_segments for physically contigous segments [Linux 5.2]

block: don’t decrement nr_phys_segments for physically contigous segments [Linux 5.2]

This Linux kernel change "block: don’t decrement nr_phys_segments for physically contigous segments" is included in the Linux 5.2 release. This change is authored by Christoph Hellwig <hch [at] lst.de> on Tue May 21 09:01:40 2019 +0200. The commit for this change in Linux stable tree is eded341 (patch).

block: don't decrement nr_phys_segments for physically contigous segments

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 can for example mistrigger the single segment
optimization in the nvme-pci driver, and might lead to mismatching
nr_phys_segments number when recalculating the number of request
when inserting a cloned request.

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.

dff824b2aadb ("nvme-pci: optimize mapping of small single segment requests").
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

There are 23 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 21e87a7..80a5a0f 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))

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