On 10/22/18 5:00 PM, Christoph Hellwig wrote:

On Sat, Oct 20, 2018 at 10:29:37PM +0800, Jianchao Wang wrote:

quoted

There are two cases when handle DISCARD merge
 - max_discard_segments == 1
   bios need to be contiguous
 - max_discard_segments > 1
   Only nvme right now. It takes every bio as a range and different
   range needn't to be contiguous.

But now, attempt_merge screws this up. It always consider contiguity
for DISCARD for the case max_discard_segments > 1 and cannot merge
contiguous DISCARD for the case max_discard_segments == 1, because
rq_attempt_discard_merge always returns false in this case.
This patch fixes both of the two cases above.

Signed-off-by: Jianchao Wang <redacted>
---

V2:
  - Add max_discard_segments > 1 checking in attempt_merge
  - Change patch title and comment
  - Add more comment in attempt_merge

 block/blk-merge.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 42a4674..8f22374 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c

@@ -734,8 +734,15 @@ static struct request *attempt_merge(struct request_queue *q,
 	/*
 	 * not contiguous
 	 */
-	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
-		return NULL;
+	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next)) {
+		/*
+		 * When max_discard_segments is bigger than 1 (only nvme right
+		 * now), needn't consider the contiguity.
+		 */
+		if (!(req_op(req) == REQ_OP_DISCARD &&
+		      queue_max_discard_segments(q) > 1))
+			return NULL;

Why not:

		if (req_op(req) != REQ_OP_DISCARD ||
		    queue_max_discard_segments(q) == 1)> 
which would be a lot more obvious?

OK, I will change it

quoted

+	 * counts here.
+	 * Two cases of Handling DISCARD:
+	 *  - max_discard_segments == 1
+	 *    The bios need to be contiguous.
+	 *  - max_discard_segments > 1
+	 *    Only nvme right now. It takes every bio as a
+	 *    range and send them to controller together. The ranges
+	 *    needn't to be contiguous.

The formatting looks odd.  Also I don't think we should mention the
users here, as others might grow (virtio is in the pipe, SCSI could
be supported easily if someone did the work).

Yes, I will change the format and discard the users here.

Thanks
Jianchao