[dm-devel] blk-mq request allocation stalls

[Jens Axboe]

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.

-- 
Jens Axboe