Re: WARNING: CPU: 2 PID: 207 at drivers/nvme/host/core.c:527 nvme_setup_cmd+0x3d3

Jens Axboe
2018-02-01T17:58:23Z

On 2/1/18 8:26 AM, Jens Axboe wrote:
> On 1/31/18 9:56 PM, Keith Busch wrote:
>> On Wed, Jan 31, 2018 at 08:07:41PM -0700, Jens Axboe wrote:
>>> if (total_phys_segments > queue_max_segments(q))
>>> - return 0;
>>> + return 0;
>>
>> This perhaps unintended change happens to point out another problem:
>> queue_max_segments is the wrong limit for discards, which require
>> queue_max_discard_segments.
>>
>> It might be easier to merge discard requests special, like how merging
>> a discard bio is handled (untested).
>
> Yeah agreed, we should just split it up completely instead instead of
> special casing it in the read/write path.
>
>
>> diff --git a/block/blk-merge.c b/block/blk-merge.c
>> index 8452fc7164cc..01671e1373ff 100644
>> --- a/block/blk-merge.c
>> +++ b/block/blk-merge.c
>> @@ -550,6 +550,28 @@ static bool req_no_special_merge(struct request *req)
>> return !q->mq_ops && req->special;
>> }
>>
>> +static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
>> + struct request *next)
>> +{
>> + 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(next->bio) >
>> + blk_rq_get_max_sectors(req, blk_rq_pos(req)))
>> + goto no_merge;
>> +
>> + req->biotail->bi_next = next->bio;
>> + req->biotail = next->biotail;
>> + req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
>> + next->bio = NULL;
>> + return true;
>> +
>> +no_merge:
>> + req_set_nomerge(q, req);
>> + return false;
>> +}
>> +
>> static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
>> struct request *next)
>> {
>> @@ -679,6 +701,15 @@ static struct request *attempt_merge(struct request_queue *q,
>> if (req->write_hint != next->write_hint)
>> return NULL;
>>
>> + /*
>> + * Discards are ... special.
>> + */
>> + if (req_op(req) == REQ_OP_DISCARD) {
>> + if (req_attempt_discard_merge(q, req, next))
>> + return next;
>> + return NULL;
>> + }
>> +
>> /*
>> * If we are allowed to merge, then append bio list
>> * from next to rq and release next. merge_requests_fn
>
> This looks fine to me, the bio-to-request merge path already looks correct.
> Care to send a properly formatted patch?

I was able to reproduce on a test box, pretty trivially in fact:

# echo mq-deadline > /sys/block/nvme2n1/queue/scheduler
# mkfs.ext4 /dev/nvme2n1
# mount /dev/nvme2n1 /data -o discard
# dd if=/dev/zero of=/data/10g bs=1M count=10k
# sync
# rm /data/10g
# sync <- triggered

Your patch still doesn't work, but mainly because we init the segments
to 0 when setting up a discard. The below works for me, and cleans up
the merge path a bit, since your patch was missing various adjustments
on both the merged and freed request.

diff --git a/block/blk-core.c b/block/blk-core.c
index a2005a485335..e4561c95fc23 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -3282,6 +3282,8 @@ void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
{
if (bio_has_data(bio))
rq->nr_phys_segments = bio_phys_segments(q, bio);
+ else if (bio_op(bio) == REQ_OP_DISCARD)
+ rq->nr_phys_segments = 1;

rq->__data_len = bio->bi_iter.bi_size;
rq->bio = rq->biotail = bio;
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8452fc7164cc..782940c65d8a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -550,6 +550,24 @@ static bool req_no_special_merge(struct request *req)
return !q->mq_ops && req->special;
}

+static bool req_attempt_discard_merge(struct request_queue *q, struct request *req,
+ struct request *next)
+{
+ 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(next->bio) >
+ blk_rq_get_max_sectors(req, blk_rq_pos(req)))
+ goto no_merge;
+
+ req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
+ return true;
+no_merge:
+ req_set_nomerge(q, req);
+ return false;
+}
+
static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
struct request *next)
{
@@ -683,9 +701,13 @@ static struct request *attempt_merge(struct request_queue *q,
* If we are allowed to merge, then append bio list
* from next to rq and release next. merge_requests_fn
* will have updated segment counts, update sector
- * counts here.
+ * counts here. Handle DISCARDs separately, as they
+ * have separate settings.
*/
- if (!ll_merge_requests_fn(q, req, next))
+ if (req_op(req) == REQ_OP_DISCARD) {
+ if (!req_attempt_discard_merge(q, req, next))
+ return NULL;
+ } else if (!ll_merge_requests_fn(q, req, next))
return NULL;

/*
@@ -715,7 +737,8 @@ static struct request *attempt_merge(struct request_queue *q,

req->__data_len += blk_rq_bytes(next);

- elv_merge_requests(q, req, next);
+ if (req_op(req) != REQ_OP_DISCARD)
+ elv_merge_requests(q, req, next);

/*
* 'next' is going away, so update stats accordingly

--
Jens Axboe