Re: [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments

[Ming Lei <ming.lei@redhat.com>]

On Mon, May 20, 2019 at 01:11:41PM +0200, Christoph Hellwig wrote:
> On Thu, May 16, 2019 at 09:17:04PM +0800, Ming Lei wrote:
> > ll_merge_requests_fn() is only called from attempt_merge() in case
> > that ELEVATOR_BACK_MERGE is returned from blk_try_req_merge(). However,
> > for discard merge of both virtio_blk and nvme, ELEVATOR_DISCARD_MERGE is
> > always returned from blk_try_req_merge() in attempt_merge(), so looks
> > ll_merge_requests_fn() shouldn't be called for virtio_blk/nvme's discard
> > request. Just wondering if you may explain a bit how the change on
> > ll_merge_requests_fn() in this patch makes a difference on the above
> > two commits?
> 
> Good question.  I've seen virtio overwriting its range, but I think
> this might have been been with a series to actually decrement
> nr_phys_segments for all cases where we can merge the tail and front
> bvecs.  So mainline probably doesn't see it unless someone calls
> blk_recalc_rq_segments due to a partial completion or when using
> dm-multipath.  Thinking of it at least the latter seems like a real
> possibily, although a rather unlikely use case.

This patch shouldn't effect discard IO in case of partial completion too
cause blk_recalc_rq_segments() always return 0 for discard IO w/wo this
patch.

However looks this way is wrong, the following patch may help for this
case:

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 1aafeb923e7b..302667887ea1 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1109,7 +1109,12 @@ static inline unsigned short blk_rq_nr_phys_segments(struct request *rq)
  */
 static inline unsigned short blk_rq_nr_discard_segments(struct request *rq)
 {
-	return max_t(unsigned short, rq->nr_phys_segments, 1);
+	struct bio *bio;
+	unsigned shart segs = 0;
+
+	__rq_for_each_bio(bio, rq)
+		segs++;
+	return segs;
 }
 
 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);

Or re-calculate the segment number in this way for multi-range discard IO in
__blk_recalc_rq_segments().

Thanks,
Ming