linux-kernel - Re: [PATCH 2/2] mm: do not loop over ALLOC_NO

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Message-ID: <20151118091101.GA19145@dhcp22.suse.cz>
Date:	Wed, 18 Nov 2015 10:11:01 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: do not loop over ALLOC_NO_WATERMARKS without
 triggering reclaim

On Tue 17-11-15 19:58:09, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > __alloc_pages_slowpath is looping over ALLOC_NO_WATERMARKS requests if
> > __GFP_NOFAIL is requested. This is fragile because we are basically
> > relying on somebody else to make the reclaim (be it the direct reclaim
> > or OOM killer) for us. The caller might be holding resources (e.g.
> > locks) which block other other reclaimers from making any progress for
> > example. Remove the retry loop and rely on __alloc_pages_slowpath to
> > invoke all allowed reclaim steps and retry logic.
> 
> This implies invoking OOM killer, doesn't it?

It does and the changelog is explicit about this.

> >   	/* Avoid recursion of direct reclaim */
> > -	if (current->flags & PF_MEMALLOC)
> > +	if (current->flags & PF_MEMALLOC) {
> > +		/*
> > +		 * __GFP_NOFAIL request from this context is rather bizarre
> > +		 * because we cannot reclaim anything and only can loop waiting
> > +		 * for somebody to do a work for us.
> > +		 */
> > +		if (WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
> > +			cond_resched();
> > +			goto retry;
> 
> I think that this "goto retry;" omits call to out_of_memory() which is allowed
> for __GFP_NOFAIL allocations. 

It wasn't called for PF_MEMALLOC requests though. Whether invoking OOM
killer is a good idea for this case is a harder question and out of
scope of this patch.

> Even if this is what you meant, current thread
> can be a workqueue, which currently need a short sleep (as with
> wait_iff_congested() changes), can't it?

As the changelog tries to clarify PF_MEMALLOC with __GFP_NOFAIL is
basically a bug. That is the reason I am adding WARN_ON there. I do not
think making this code more complex for abusers/buggy code is really
worthwhile. Besides that I fail to see why a work item would ever
want to set PF_MEMALLOC for legitimate reasons. I have done a quick git
grep over the tree and there doesn't seem to be any user.

> 
> > +		}
> >   		goto nopage;
> > +	}
> >   
> >   	/* Avoid allocations with no watermarks from looping endlessly */
> >   	if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
> > 
> 
> Well, is it cond_resched() which should include
> 
>   if (current->flags & PF_WQ_WORKER)
>   	schedule_timeout(1);

I believe you are getting off-topic here.
-- 
Michal Hocko
SUSE Labs
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