linux-kernel - Re: [PATCH] mm: avoid livelock on !__GFP

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Date:	Tue, 15 Nov 2011 10:42:23 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Colin Cross <ccross@...roid.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

On Mon, Nov 14, 2011 at 03:03:26PM -0800, Andrew Morton wrote:
> > <SNIP>
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 9dd443d..5402897 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -127,6 +127,20 @@ void pm_restrict_gfp_mask(void)
> >  	saved_gfp_mask = gfp_allowed_mask;
> >  	gfp_allowed_mask &= ~GFP_IOFS;
> >  }
> > +
> > +static bool pm_suspending(void)
> > +{
> > +	if ((gfp_allowed_mask & GFP_IOFS) == GFP_IOFS)
> > +		return false;
> > +	return true;
> > +}
> 
> This doesn't seem a terribly reliable way of detecting that PM has
> disabled the storage devices (which is what we really want to know
> here: kswapd got crippled).
> 

It only works because PM is the only caller that alters
gfp_allowed_mask at runtime after early boot completes. We also check
if suspend is in progress in mm/swapfile.c#try_to_free_swap() using
the gfp_allowed_mask.

> I guess it's safe for now, because PM is the only caller who alters
> gfp_allowed_mask (I assume). 

You assume correctly.

> But an explicit storage_is_unavaliable
> global which is set and cleared at exactly the correct time is clearer,
> more direct and future-safer, no?
> 

It feels overkill to allocate more global storage for it when
gfp_allowed_mask is already there but I could rename pm_suspending() to
pm_disabled_storage(), make try_to_free_swap() use the same helper but
leave the implementation the same. This would clarify the situation.

> > +#else
> > +
> > +static bool pm_suspending(void)
> > +{
> > +	return false;
> > +}
> >  #endif /* CONFIG_PM_SLEEP */
> >  
> >  #ifdef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
> > @@ -2214,6 +2228,14 @@ rebalance:
> >  
> >  			goto restart;
> >  		}
> > +
> > +		/*
> > +		 * Suspend converts GFP_KERNEL to __GFP_WAIT which can
> > +		 * prevent reclaim making forward progress without
> > +		 * invoking OOM. Bail if we are suspending
> > +		 */
> > +		if (pm_suspending())
> > +			goto nopage;
> 
> The comment doesn't tell the whole story: it's important that kswapd
> writeout was disabled?
> 

Writeout is disabled for flushers as well but your comment covers both
and clarifies the situation. Thanks

-- 
Mel Gorman
SUSE Labs
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