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

Keith Busch
2018-02-01T04:56:45Z

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).

---
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
--