block: don't use bio_split_rw on misc operations

bio_split_rw is designed to split read and write bios with a payload.
Currently it is called by __bio_split_to_limits for all operations not
explicitly list, which works because bio_may_need_split explicitly checks
for bi_vcnt == 1 and thus skips the bypass if there is no payload and
bio_for_each_bvec loop will never execute it's body if bi_size is 0.

But all this is hard to understand, fragile and wasted pointless cycles.
Switch __bio_split_to_limits to only call bio_split_rw for READ and
WRITE command and don't attempt any kind split for operation that do not
require splitting.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Tested-by: Hans Holmberg <[email protected]>
Reviewed-by: Hans Holmberg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Christoph Hellwig

block/blk.h

@@ -372,7 +372,8 @@ 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:
+	case REQ_OP_READ:
+	case REQ_OP_WRITE:
 		if (bio_may_need_split(bio, lim))
 			return bio_split_rw(bio, lim, nr_segs);
 		*nr_segs = 1;
@@ -384,6 +385,10 @@ static inline struct bio *__bio_split_to_limits(struct bio *bio,
 		return bio_split_discard(bio, lim, nr_segs);
 	case REQ_OP_WRITE_ZEROES:
 		return bio_split_write_zeroes(bio, lim, nr_segs);
+	default:
+		/* other operations can't be split */
+		*nr_segs = 0;
+		return bio;
 	}
 }