Re: blk-mq request allocation stalls

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Jan 13 2015 at  9:59am -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> On 01/12/2015 11:35 AM, Keith Busch wrote:
> >On Mon, 12 Jan 2015, Keith Busch wrote:
> >>Oh, let's look at "__blk_rq_prep_clone". dm calls that after
> >>blk_get_request() for the blk-mq based multipath types and overrides the
> >>destinations cmd_flags with the source's even though the source was not
> >>allocated from a blk-mq based queue, much less a shared tag.
> >
> >Untested patch. This will also preserve the failfast cmd_flag dm-mpath
> >set after allocating.
> >
> >---
> >diff --git a/block/blk-core.c b/block/blk-core.c
> >index 7e78931..6201090 100644
> >--- a/block/blk-core.c
> >+++ b/block/blk-core.c
> >@@ -2895,7 +2895,10 @@ EXPORT_SYMBOL_GPL(blk_rq_unprep_clone);
> >  static void __blk_rq_prep_clone(struct request *dst, struct request *src)
> >  {
> >      dst->cpu = src->cpu;
> >-    dst->cmd_flags = (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
> >+    if (dst->q->mq_ops)
> >+        dst->cmd_flags |= (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
> >+    else
> >+        dst->cmd_flags = (src->cmd_flags & REQ_CLONE_MASK) | REQ_NOMERGE;
> 
> Making the two cases different is a bit... nonsensical. We should do
> this for both cases, if safe, or move the MQ_INFLIGHT flag and
> expand the CLONE_MASK.

ok, i'll work through it.
k