alinux: block: don't decrement nr_phys_segments for physically contigous segments

fix #29327388

This patch is inspired by commit eded341c085b ("don't decrement
nr_phys_segments for physically contigous segments"). Now the
ll_merge_requests_fn() will reduces nr_phys_segments by one if the
last segment of the previous, and the first segment of the next
segment are contigous. But that will mismatch the segment numbers
when handling discard request for virtio_block and nvme, moreover
if nvme queue selects IO scheduler, that also will mistrigger the
single segment optimization in the nvme-pci driver. Thus we should
not decrement nr_phys_segments in ll_merge_requests_fn().

But in __blk_recalc_rq_segments(), if we re-calculate the segments of
one request, we may get a different segment number, due to it will merge
physically contigous segments here. Originally we should simply remove
the bvec merging optimization under the assumption that most users already
build good enough bvecs, but we do not want to touch so many core logics
here, which may introduce more issues. So we can simply not allow to merge
contigous segments between 2 bios to fix this issue, and this is not in
the fast path, which will not affect the performance.

Fixes: dff824b2aadb ("nvme-pci: optimize mapping of small single segment requests").
Signed-off-by: NBaolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

block/blk-merge.c

@@ -288,6 +288,7 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
 			seg_size = bv.bv_len;
 		}
 		bbio = bio;
+		prev = 0;
 	}
 
 	if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
@@ -603,7 +604,6 @@ 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))