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@xxxxxxxxx>
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);
