[dm-devel] blk mq: don't blk_mq_request_bypass_insert _and_ return BLK_STS_RESOURCE

[Mike Snitzer]

On Wed, Jan 17 2018 at 10:39pm -0500,
Ming Lei <ming.lei at redhat.com> wrote:

> On Wed, Jan 17, 2018 at 10:33:35PM -0500, Mike Snitzer wrote:
> > On Wed, Jan 17 2018 at  7:54P -0500,
> > Mike Snitzer <snitzer at redhat.com> wrote:
> >  
> > > But sure, I suppose there is something I missed when refactoring Ming's
> > > change to get it acceptable for upstream.  I went over the mechanical
> > > nature of what I did many times (comparing Ming's v4 to my v5).
> > 
> > And yes there is one subtlety that I missed.
> > 
> > > The call to blk_mq_request_bypass_insert will only occur via
> > > __blk_mq_fallback_to_insert.  Which as the name implies this is not the
> > > fast path.  This will occur if the underlying blk-mq device cannot get
> > > resources it needs in order to issue the request.  Specifically: if/when
> > > in __blk_mq_try_issue_directly() the hctx is stopped, or queue is
> > > quiesced, or it cannot get the driver tag or dispatch_budget (in the
> > > case of scsi-mq).
> > > 
> > > The same fallback, via call to blk_mq_request_bypass_insert, occured
> > > with Ming's v4 though.
> > 
> > Turns out Ming's v4 doesn't fallback to insert for the "or it cannot get
> > the driver tag or dispatch_budget" case.
> > 
> > This patch should fix it (Laurence, please report back on if this fixes
> > your list_add corruption, pretty sure it will):
> > 
> > From: Mike Snitzer <snitzer at redhat.com>
> > Date: Wed, 17 Jan 2018 22:02:07 -0500
> > Subject: [PATCH] blk mq: don't blk_mq_request_bypass_insert _and_ return BLK_STS_RESOURCE
> > 
> > It isn't ever valid to call blk_mq_request_bypass_insert() and return
> > BLK_STS_RESOURCE.
> > 
> > Unfortunately after commit 396eaf21ee ("blk-mq: improve DM's blk-mq IO
> > merging via blk_insert_cloned_request feedback") we do just that if
> > blk_mq_request_direct_issue() cannot get the resources (driver_tag or
> > dispatch_budget) needed to directly issue a request.  This will lead to
> > "list_add corruption" because blk-mq submits the IO but then reports
> > that it didn't (BLK_STS_RESOURCE in this case).
> > 
> > Fix this by simply returning BLK_STS_RESOURCE for this case.
> > 
> > Fixes: 396eaf21ee ("blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback")
> > Reported-by: Laurence Oberman <loberman at redhat.com>
> > Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> > ---
> >  block/blk-mq.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index c418858a60ef..8bee37239255 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -1799,20 +1799,18 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> >  	if (q->elevator && !bypass_insert)
> >  		goto insert;
> >  
> > -	if (!blk_mq_get_driver_tag(rq, NULL, false))
> > -		goto insert;
> > -
> > -	if (!blk_mq_get_dispatch_budget(hctx)) {
> > +	if (!blk_mq_get_driver_tag(rq, NULL, false) ||
> > +	    !blk_mq_get_dispatch_budget(hctx)) {
> > +		/* blk_mq_put_driver_tag() is idempotent */
> >  		blk_mq_put_driver_tag(rq);
> > +		if (bypass_insert)
> > +			return BLK_STS_RESOURCE;
> >  		goto insert;
> >  	}
> >  
> >  	return __blk_mq_issue_directly(hctx, rq, cookie);
> >  insert:
> >  	__blk_mq_fallback_to_insert(rq, run_queue, bypass_insert);
> > -	if (bypass_insert)
> > -		return BLK_STS_RESOURCE;
> > -
> >  	return BLK_STS_OK;
> >  }
> 
> Hi Mike,
> 
> I'd suggest to use the following one, which is simple and clean:
> 
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 4d4af8d712da..816ff5d6bc88 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1856,15 +1856,6 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
>  	return ret;
>  }
>  
> -static void __blk_mq_fallback_to_insert(struct request *rq,
> -					bool run_queue, bool bypass_insert)
> -{
> -	if (!bypass_insert)
> -		blk_mq_sched_insert_request(rq, false, run_queue, false);
> -	else
> -		blk_mq_request_bypass_insert(rq, run_queue);
> -}
> -
>  static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  						struct request *rq,
>  						blk_qc_t *cookie,
> @@ -1892,10 +1883,10 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  
>  	return __blk_mq_issue_directly(hctx, rq, cookie);
>  insert:
> -	__blk_mq_fallback_to_insert(rq, run_queue, bypass_insert);
>  	if (bypass_insert)
>  		return BLK_STS_RESOURCE;
>  
> +	blk_mq_sched_insert_request(rq, false, run_queue, false);
>  	return BLK_STS_OK;
>  }
>  
> @@ -1911,7 +1902,7 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
>  
>  	ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false);
>  	if (ret == BLK_STS_RESOURCE)
> -		__blk_mq_fallback_to_insert(rq, true, false);
> +		blk_mq_sched_insert_request(rq, false, true, false);
>  	else if (ret != BLK_STS_OK)
>  		blk_mq_end_request(rq, ret);
>  

That'd be another way to skin the cat.. BUT it is different than your
original approach (due to the case I detailed in earlier mail).

But I like the simplicity of always returning BLK_STS_RESOURCE.

Reviewed-by: Mike Snitzer <snitzer at redhat.com>

Laurance, please test Ming's patch instead.  Ming, if Laurance finds
this fix works (which is should) please put a formal header on the patch
and submit for Jens to pick up.  Sorry about screwing this up.

(feel free to re-use portions of my above patch header to help explain
the problem in your header)

Thanks,
Mike