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

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Date:	Tue, 25 Oct 2011 22:47:15 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Colin Cross <ccross@...roid.com>
cc:	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mm: avoid livelock on !__GFP_FS allocations

On Tue, 25 Oct 2011, Colin Cross wrote:

> > On Tue, 25 Oct 2011, Mel Gorman wrote:
> >
> >> That said, it will be difficult to remember why checking __GFP_NOFAIL in
> >> this case is necessary and someone might "optimitise" it away later. It
> >> would be preferable if it was self-documenting. Maybe something like
> >> this? (This is totally untested)
> >>
> >
> > __GFP_NOFAIL _should_ be optimized away in this case because all he's
> > passing is __GFP_WAIT | __GFP_NOFAIL.  That doesn't make any sense unless
> > all you want to do is livelock.
> 
> __GFP_NOFAIL is not set in the case that I care about.  If my change
> is hit, no forward progress has been made, so I agree it should not
> honor __GFP_NOFAIL.
> 

I was responding to Mel's comment, not your case.

> > __GFP_NOFAIL doesn't mean the page allocator would infinitely loop in all
> > conditions.  That's why GFP_ATOMIC | __GFP_NOFAIL actually fails, and I
> > would argue that __GFP_WAIT | __GFP_NOFAIL should fail as well since it's
> > the exact same condition except doesn't have access to the extra memory
> > reserves.
> >
> > Suspend needs to either set __GFP_NORETRY to avoid the livelock if it's
> > going to disable all means of memory reclaiming or freeing in the page
> > allocator.  Or, better yet, just make it GFP_NOWAIT.
> >
> 
> It would be nice to give compaction and the slab shrinker a chance to
> recover a few pages, both methods will work fine in suspend.

Ok, so __GFP_NORETRY it is.  Just make sure that when 
pm_restrict_gfp_mask() masks off __GFP_IO and __GFP_FS that it also sets 
__GFP_NORETRY even though the name of the function no longer seems 
appropriate anymore.

> GFP_NOWAIT will prevent them from ever running, and __GFP_NORETRY will
> give up even if they are making progress but haven't recovered enough
> pages.
> 

These are all order-3 or smaller allocations where fragmentation isn't a 
big issue.  If a call into direct compaction or reclaim fails to reclaim 
that small amount of contiguous memory, what makes you believe that a 
second call will?