block: change the blk_queue_split calling convention

The double indirect bio leads to somewhat suboptimal code generation.
Instead return the (original or split) bio, and make sure the
request_queue arguments to the lower level helpers is passed after the
bio to avoid constant reshuffling of the argument passing registers.

Also give it and the helpers used to implement it more descriptive names.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220727162300.3089193-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

3 files changed, 53 insertions, 55 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 4c8a699754c9..6e29fb28584e 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -95,10 +95,8 @@ static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
 	return bio_will_gap(req->q, NULL, bio, req->bio);
 }
 
-static struct bio *blk_bio_discard_split(struct request_queue *q,
-					 struct bio *bio,
-					 struct bio_set *bs,
-					 unsigned *nsegs)
+static struct bio *bio_split_discard(struct bio *bio, struct request_queue *q,
+		unsigned *nsegs, struct bio_set *bs)
 {
 	unsigned int max_discard_sectors, granularity;
 	int alignment;
@@ -139,8 +137,8 @@ static struct bio *blk_bio_discard_split(struct request_queue *q,
 	return bio_split(bio, split_sectors, GFP_NOIO, bs);
 }
 
-static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
-		struct bio *bio, struct bio_set *bs, unsigned *nsegs)
+static struct bio *bio_split_write_zeroes(struct bio *bio,
+		struct request_queue *q, unsigned *nsegs, struct bio_set *bs)
 {
 	*nsegs = 0;
 
@@ -161,8 +159,8 @@ static struct bio *blk_bio_write_zeroes_split(struct request_queue *q,
  * requests that are submitted to a block device if the start of a bio is not
  * aligned to a physical block boundary.
  */
-static inline unsigned get_max_io_size(struct request_queue *q,
-				       struct bio *bio)
+static inline unsigned get_max_io_size(struct bio *bio,
+		struct request_queue *q)
 {
 	unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
 	unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
@@ -247,16 +245,16 @@ static bool bvec_split_segs(const struct request_queue *q,
 }
 
 /**
- * blk_bio_segment_split - split a bio in two bios
- * @q:    [in] request queue pointer
+ * bio_split_rw - split a bio in two bios
  * @bio:  [in] bio to be split
- * @bs:	  [in] bio set to allocate the clone from
+ * @q:    [in] request queue pointer
  * @segs: [out] number of segments in the bio with the first half of the sectors
+ * @bs:	  [in] bio set to allocate the clone from
  *
  * Clone @bio, update the bi_iter of the clone to represent the first sectors
  * of @bio and update @bio->bi_iter to represent the remaining sectors. The
  * following is guaranteed for the cloned bio:
- * - That it has at most get_max_io_size(@q, @bio) sectors.
+ * - That it has at most get_max_io_size(@bio, @q) sectors.
  * - That it has at most queue_max_segments(@q) segments.
  *
  * Except for discard requests the cloned bio will point at the bi_io_vec of
@@ -265,15 +263,13 @@ static bool bvec_split_segs(const struct request_queue *q,
  * responsible for ensuring that @bs is only destroyed after processing of the
  * split bio has finished.
  */
-static struct bio *blk_bio_segment_split(struct request_queue *q,
-					 struct bio *bio,
-					 struct bio_set *bs,
-					 unsigned *segs)
+static struct bio *bio_split_rw(struct bio *bio, struct request_queue *q,
+		unsigned *segs, struct bio_set *bs)
 {
 	struct bio_vec bv, bvprv, *bvprvp = NULL;
 	struct bvec_iter iter;
 	unsigned nsegs = 0, bytes = 0;
-	const unsigned max_bytes = get_max_io_size(q, bio) << 9;
+	const unsigned max_bytes = get_max_io_size(bio, q) << 9;
 	const unsigned max_segs = queue_max_segments(q);
 
 	bio_for_each_bvec(bv, bio, iter) {
@@ -320,34 +316,33 @@ split:
 }
 
 /**
- * __blk_queue_split - split a bio and submit the second half
- * @q:       [in] request_queue new bio is being queued at
- * @bio:     [in, out] bio to be split
- * @nr_segs: [out] number of segments in the first bio
+ * __bio_split_to_limits - split a bio to fit the queue limits
+ * @bio:     bio to be split
+ * @q:       request_queue new bio is being queued at
+ * @nr_segs: returns the number of segments in the returned bio
+ *
+ * Check if @bio needs splitting based on the queue limits, and if so split off
+ * a bio fitting the limits from the beginning of @bio and return it.  @bio is
+ * shortened to the remainder and re-submitted.
  *
- * Split a bio into two bios, chain the two bios, submit the second half and
- * store a pointer to the first half in *@bio. If the second bio is still too
- * big it will be split by a recursive call to this function. Since this
- * function may allocate a new bio from q->bio_split, it is the responsibility
- * of the caller to ensure that q->bio_split is only released after processing
- * of the split bio has finished.
+ * The split bio is allocated from @q->bio_split, which is provided by the
+ * block layer.
  */
-void __blk_queue_split(struct request_queue *q, struct bio **bio,
+struct bio *__bio_split_to_limits(struct bio *bio, struct request_queue *q,
 		       unsigned int *nr_segs)
 {
-	struct bio *split = NULL;
+	struct bio *split;
 
-	switch (bio_op(*bio)) {
+	switch (bio_op(bio)) {
 	case REQ_OP_DISCARD:
 	case REQ_OP_SECURE_ERASE:
-		split = blk_bio_discard_split(q, *bio, &q->bio_split, nr_segs);
+		split = bio_split_discard(bio, q, nr_segs, &q->bio_split);
 		break;
 	case REQ_OP_WRITE_ZEROES:
-		split = blk_bio_write_zeroes_split(q, *bio, &q->bio_split,
-				nr_segs);
+		split = bio_split_write_zeroes(bio, q, nr_segs, &q->bio_split);
 		break;
 	default:
-		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
+		split = bio_split_rw(bio, q, nr_segs, &q->bio_split);
 		break;
 	}
 
@@ -356,32 +351,35 @@ void __blk_queue_split(struct request_queue *q, struct bio **bio,
 		split->bi_opf |= REQ_NOMERGE;
 
 		blkcg_bio_issue_init(split);
-		bio_chain(split, *bio);
-		trace_block_split(split, (*bio)->bi_iter.bi_sector);
-		submit_bio_noacct(*bio);
-		*bio = split;
+		bio_chain(split, bio);
+		trace_block_split(split, bio->bi_iter.bi_sector);
+		submit_bio_noacct(bio);
+		return split;
 	}
+	return bio;
 }
 
 /**
- * blk_queue_split - split a bio and submit the second half
- * @bio: [in, out] bio to be split
+ * bio_split_to_limits - split a bio to fit the queue limits
+ * @bio:     bio to be split
+ *
+ * Check if @bio needs splitting based on the queue limits of @bio->bi_bdev, and
+ * if so split off a bio fitting the limits from the beginning of @bio and
+ * return it.  @bio is shortened to the remainder and re-submitted.
  *
- * Split a bio into two bios, chains the two bios, submit the second half and
- * store a pointer to the first half in *@bio. Since this function may allocate
- * a new bio from q->bio_split, it is the responsibility of the caller to ensure
- * that q->bio_split is only released after processing of the split bio has
- * finished.
+ * The split bio is allocated from @q->bio_split, which is provided by the
+ * block layer.
  */
-void blk_queue_split(struct bio **bio)
+struct bio *bio_split_to_limits(struct bio *bio)
 {
-	struct request_queue *q = bdev_get_queue((*bio)->bi_bdev);
+	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
 	unsigned int nr_segs;
 
-	if (blk_may_split(q, *bio))
-		__blk_queue_split(q, bio, &nr_segs);
+	if (bio_may_exceed_limits(bio, q))
+		return __bio_split_to_limits(bio, q, &nr_segs);
+	return bio;
 }
-EXPORT_SYMBOL(blk_queue_split);
+EXPORT_SYMBOL(bio_split_to_limits);
 
 unsigned int blk_recalc_rq_segments(struct request *rq)
 {
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 70177ee74295..790f55453f1b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2816,8 +2816,8 @@ void blk_mq_submit_bio(struct bio *bio)
 	blk_status_t ret;
 
 	blk_queue_bounce(q, &bio);
-	if (blk_may_split(q, bio))
-		__blk_queue_split(q, &bio, &nr_segs);
+	if (bio_may_exceed_limits(bio, q))
+		bio = __bio_split_to_limits(bio, q, &nr_segs);
 
 	if (!bio_integrity_prep(bio))
 		return;
diff --git a/block/blk.h b/block/blk.h
index 1d83b1d41cd1..623be4c2e60c 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -288,7 +288,7 @@ 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 request_queue *q, struct bio *bio)
+static inline bool bio_may_exceed_limits(struct bio *bio, struct request_queue *q)
 {
 	switch (bio_op(bio)) {
 	case REQ_OP_DISCARD:
@@ -311,8 +311,8 @@ static inline bool blk_may_split(struct request_queue *q, struct bio *bio)
 		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
 }
 
-void __blk_queue_split(struct request_queue *q, struct bio **bio,
-			unsigned int *nr_segs);
+struct bio *__bio_split_to_limits(struct bio *bio, struct request_queue *q,
+		       unsigned int *nr_segs);
 int ll_back_merge_fn(struct request *req, struct bio *bio,
 		unsigned int nr_segs);
 bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,