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Subject[PATCH V3] block: fix the DISCARD request merge
Date
There are two cases when handle DISCARD merge.
If max_discard_segments == 1, the bios/requests need to be contiguous
to merge. If max_discard_segments > 1, 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 <jianchao.w.wang@oracle.com>
---

V3:
- Introduce blk_discard_mergable into attempt_merge and
blk_try_merge.
- Some comment changes.

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 | 34 ++++++++++++++++++++++++----------
1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 42a4674..b258de0 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -714,6 +714,22 @@ static void blk_account_io_merge(struct request *req)
part_stat_unlock();
}
}
+/*
+ * Two cases of handling DISCARD merge:
+ * If max_discard_segments > 1, the driver takes every bio
+ * as a range and send them to controller together. The ranges
+ * needn't to be contiguous.
+ * Otherwise, the bios/requests will be handled as same as
+ * others which should be contiguous.
+ */
+static inline bool blk_discard_mergable(struct request *req)
+{
+ if (req_op(req) == REQ_OP_DISCARD &&
+ queue_max_discard_segments(req->q) > 1)
+ return true;
+ else
+ return false;
+}

/*
* For non-mq, this has to be called with the request spinlock acquired.
@@ -731,12 +747,6 @@ static struct request *attempt_merge(struct request_queue *q,
if (req_op(req) != req_op(next))
return NULL;

- /*
- * not contiguous
- */
- if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
- return NULL;
-
if (rq_data_dir(req) != rq_data_dir(next)
|| req->rq_disk != next->rq_disk
|| req_no_special_merge(next))
@@ -760,11 +770,16 @@ static struct request *attempt_merge(struct request_queue *q,
* counts here. Handle DISCARDs separately, as they
* have separate settings.
*/
- if (req_op(req) == REQ_OP_DISCARD) {
+
+ if (blk_discard_mergable(req)) {
if (!req_attempt_discard_merge(q, req, next))
return NULL;
- } else if (!ll_merge_requests_fn(q, req, next))
+ } else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next)) {
+ if (!ll_merge_requests_fn(q, req, next))
+ return NULL;
+ } else {
return NULL;
+ }

/*
* If failfast settings disagree or any of the two is already
@@ -888,8 +903,7 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)

enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
{
- if (req_op(rq) == REQ_OP_DISCARD &&
- queue_max_discard_segments(rq->q) > 1)
+ if (blk_discard_mergable(rq))
return ELEVATOR_DISCARD_MERGE;
else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
return ELEVATOR_BACK_MERGE;
--
2.7.4
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