On 1/31/18 8:03 PM, jianchao.wang wrote:
> Hi Jens
>
>
> On 01/31/2018 11:29 PM, Jens Axboe wrote:
>> How about something like the below?
>>
>>
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index 8452fc7164cc..cee102fb060e 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -574,8 +574,13 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
>> blk_rq_get_max_sectors(req, blk_rq_pos(req)))
>> return 0;
>>
>> + /*
>> + * For DISCARDs, the segment count isn't interesting since
>> + * the requests have no data attached.
>> + */
>> total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
>> - if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
>> + if (total_phys_segments &&
>> + blk_phys_contig_segment(q, req->biotail, next->bio)) {
>> if (req->nr_phys_segments == 1)
>> req->bio->bi_seg_front_size = seg_size;
>> if (next->nr_phys_segments == 1)
>
> This patch will avoid the nr_phys_segments to be set to 0xffff,
> but the merged req will have two bios but zero nr_phys_segments.
>
> We have to align with the DISCARD merging strategy.
>
> Please refer to:
> /*
> * Number of discard segments (or ranges) the driver needs to fill in.
> * Each discard bio merged into a request is counted as one segment.
> */
> static inline unsigned short blk_rq_nr_discard_segments(struct request *rq)
> {
> return max_t(unsigned short, rq->nr_phys_segments, 1);
> }
> bool bio_attempt_discard_merge(struct request_queue *q, struct request *req,
> struct bio *bio)
> {
> unsigned short segments = blk_rq_nr_discard_segments(req);
>
> if (segments >= queue_max_discard_segments(q))
> goto no_merge;
> if (blk_rq_sectors(req) + bio_sectors(bio) >
> blk_rq_get_max_sectors(req, blk_rq_pos(req)))
> goto no_merge;
>
> req->biotail->bi_next = bio;
> req->biotail = bio;
> req->__data_len += bio->bi_iter.bi_size;
> req->ioprio = ioprio_best(req->ioprio, bio_prio(bio));
> req->nr_phys_segments = segments + 1;
>
> blk_account_io_start(req, false);
> return true;
> no_merge:
> req_set_nomerge(q, req);
> return false;
> }
>
> blk_rq_nr_discard_segments will get a wrong value finally.
>
> Maybe we could change blk_rq_nr_discard_segments to iterate and count the bios in one request
> to decide the DISCARD request nr_phy_segment. And discard the nr_phys_segments operations in
> the DISCARD merging path, plus your patch here.
Yeah I agree, and my last patch missed that we do care about segments for
discards. Below should be better...
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8452fc7164cc..055057bd727f 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -553,9 +553,8 @@ static bool req_no_special_merge(struct request *req)
static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
struct request *next)
{
- int total_phys_segments;
- unsigned int seg_size =
- req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
+ int total_phys_segments = req->nr_phys_segments +
+ next->nr_phys_segments;
/*
* First check if the either of the requests are re-queued
@@ -574,8 +573,15 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
blk_rq_get_max_sectors(req, blk_rq_pos(req)))
return 0;
- total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
- if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
+ /*
+ * If the requests aren't carrying any data payloads, we don't need
+ * to look at the segment count
+ */
+ if (bio_has_data(next->bio) &&
+ blk_phys_contig_segment(q, req->biotail, next->bio)) {
+ unsigned int seg_size = req->biotail->bi_seg_back_size +
+ next->bio->bi_seg_front_size;
+
if (req->nr_phys_segments == 1)
req->bio->bi_seg_front_size = seg_size;
if (next->nr_phys_segments == 1)
@@ -584,7 +590,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
}
if (total_phys_segments > queue_max_segments(q))
- return 0;
+ return 0;
if (blk_integrity_merge_rq(q, req, next) == false)
return 0;
--
Jens Axboe
