block: rework bio splitting

The current setup with bio_may_exceed_limit and __bio_split_to_limits
is a bit of a mess.

Change it so that __bio_split_to_limits does all the work and is just
a variant of bio_split_to_limits that returns nr_segs.  This is done
by inlining it and instead have the various bio_split_* helpers directly
submit the potentially split bios.

To support btrfs, the rw version has a lower level helper split out
that just returns the offset to split.  This turns out to nicely clean
up the btrfs flow as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Link: https://lore.kernel.org/r/20240826173820.1690925-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 44 insertions, 19 deletions

diff --git a/block/blk.h b/block/blk.h
index e180863f918b..0d8cd64c1260 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -331,33 +331,58 @@ 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 bio_may_exceed_limits(struct bio *bio,
-					 const struct queue_limits *lim)
+struct bio *bio_split_discard(struct bio *bio, const struct queue_limits *lim,
+		unsigned *nsegs);
+struct bio *bio_split_write_zeroes(struct bio *bio,
+		const struct queue_limits *lim, unsigned *nsegs);
+struct bio *bio_split_rw(struct bio *bio, const struct queue_limits *lim,
+		unsigned *nr_segs);
+
+/*
+ * All drivers must accept single-segments bios that are smaller than 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
+ * positives when bios are cloned, but compared to the performance impact of
+ * cloned bios themselves the loop below doesn't matter anyway.
+ */
+static inline bool bio_may_need_split(struct bio *bio,
+		const struct queue_limits *lim)
+{
+	return lim->chunk_sectors || bio->bi_vcnt != 1 ||
+		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
+}
+
+/**
+ * __bio_split_to_limits - split a bio to fit the queue limits
+ * @bio:     bio to be split
+ * @lim:     queue limits to split based on
+ * @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.
+ *
+ * The split bio is allocated from @q->bio_split, which is provided by the
+ * block layer.
+ */
+static inline struct bio *__bio_split_to_limits(struct bio *bio,
+		const struct queue_limits *lim, unsigned int *nr_segs)
 {
 	switch (bio_op(bio)) {
+	default:
+		if (bio_may_need_split(bio, lim))
+			return bio_split_rw(bio, lim, nr_segs);
+		*nr_segs = 1;
+		return bio;
 	case REQ_OP_DISCARD:
 	case REQ_OP_SECURE_ERASE:
+		return bio_split_discard(bio, lim, nr_segs);
 	case REQ_OP_WRITE_ZEROES:
-		return true; /* non-trivial splitting decisions */
-	default:
-		break;
+		return bio_split_write_zeroes(bio, lim, nr_segs);
 	}
-
-	/*
-	 * 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 lim->chunk_sectors || bio->bi_vcnt != 1 ||
-		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
 }
 
-struct bio *__bio_split_to_limits(struct bio *bio,
-				  const struct queue_limits *lim,
-				  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,