Re: [PATCH] block: handle fast path of bio splitting inline

[Christoph Hellwig <hch@infradead.org>]

On Wed, Oct 13, 2021 at 02:44:14PM -0600, Jens Axboe wrote:
> The fast path is no splitting needed. Separate the handling into a
> check part we can inline, and an out-of-line handling path if we do
> need to split.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

What about something like this version instead?

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 9a55b50708293..f333afb45eb15 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -353,21 +353,6 @@ void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
 				nr_segs);
 		break;
 	default:
-		/*
-		 * All drivers must accept single-segments bios that are <=
-		 * PAGE_SIZE.  This is a quick and dirty check that relies on
-		 * the fact that bi_io_vec[0] is always valid if a bio has data.
-		 * The check might lead to occasional false negatives when bios
-		 * are cloned, but compared to the performance impact of cloned
-		 * bios themselves the loop below doesn't matter anyway.
-		 */
-		if (!q->limits.chunk_sectors &&
-		    (*bio)->bi_vcnt == 1 &&
-		    ((*bio)->bi_io_vec[0].bv_len +
-		     (*bio)->bi_io_vec[0].bv_offset) <= PAGE_SIZE) {
-			*nr_segs = 1;
-			break;
-		}
 		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
 		break;
 	}
@@ -399,7 +384,8 @@ void blk_queue_split(struct bio **bio)
 {
 	unsigned int nr_segs;
 
-	__blk_queue_split(bio, &nr_segs);
+	if (blk_may_split(*bio))
+		__blk_queue_split(bio, &nr_segs);
 }
 EXPORT_SYMBOL(blk_queue_split);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 608b270a7f6b8..7c82e052ca83f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2251,11 +2251,12 @@ void blk_mq_submit_bio(struct bio *bio)
 	const int is_flush_fua = op_is_flush(bio->bi_opf);
 	struct request *rq;
 	struct blk_plug *plug;
-	unsigned int nr_segs;
+	unsigned int nr_segs = 1;
 	blk_status_t ret;
 
 	blk_queue_bounce(q, &bio);
-	__blk_queue_split(&bio, &nr_segs);
+	if (blk_may_split(bio))
+		__blk_queue_split(&bio, &nr_segs);
 
 	if (!bio_integrity_prep(bio))
 		goto queue_exit;
diff --git a/block/blk.h b/block/blk.h
index 0afee3e6a7c1e..34b31baf51324 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -262,6 +262,31 @@ ssize_t part_timeout_show(struct device *, struct device_attribute *, char *);
 ssize_t part_timeout_store(struct device *, struct device_attribute *,
 				const char *, size_t);
 
+static inline bool blk_may_split(struct bio *bio)
+{
+	switch (bio_op(bio)) {
+	case REQ_OP_DISCARD:
+	case REQ_OP_SECURE_ERASE:
+	case REQ_OP_WRITE_ZEROES:
+	case REQ_OP_WRITE_SAME:
+		return true; /* non-trivial splitting decisions */
+	default:
+		break;
+	}
+
+	/*
+	 * All drivers must accept single-segments bios that are <= PAGE_SIZE.
+	 * This is a quick and dirty check that relies on the fact that
+	 * bi_io_vec[0] is always valid if a bio has data.  The check might
+	 * lead to occasional false negatives when bios are cloned, but compared
+	 * to the performance impact of cloned bios themselves the loop below
+	 * doesn't matter anyway.
+	 */
+	return bio->bi_bdev->bd_disk->queue->limits.chunk_sectors ||
+		bio->bi_vcnt != 1 ||
+		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset >PAGE_SIZE;
+}
+
 void __blk_queue_split(struct bio **bio, unsigned int *nr_segs);
 int ll_back_merge_fn(struct request *req, struct bio *bio,
 		unsigned int nr_segs);