Re: [PATCH V3] block: optimize for small block size IO

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

I still haven't seen an explanation why this simple thing doesn't work
just as well:

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 48e6725b32ee..f3073700166f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -309,6 +309,11 @@ void __blk_queue_split(struct request_queue *q, struct bio **bio,
 				nr_segs);
 		break;
 	default:
+		if ((*bio)->bi_vcnt == 1 &&
+		    (*bio)->bi_io_vec[0].bv_len <= PAGE_SIZE) {
+			*nr_segs = 1;
+			return;
+		}
 		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
 		break;
 	}