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

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Date:	Tue, 25 Oct 2011 23:57:04 -0700
From:	Colin Cross <ccross@...roid.com>
To:	David Rientjes <rientjes@...gle.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, Oct 25, 2011 at 11:51 PM, David Rientjes <rientjes@...gle.com> wrote:
> On Tue, 25 Oct 2011, Colin Cross wrote:
>
>> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> >> index fef8dc3..59cd4ff 100644
>> >> --- a/mm/page_alloc.c
>> >> +++ b/mm/page_alloc.c
>> >> @@ -1786,6 +1786,13 @@ should_alloc_retry(gfp_t gfp_mask, unsigned int order,
>> >>                 return 0;
>> >>
>> >>         /*
>> >> +        * If PM has disabled I/O, OOM is disabled and reclaim is unlikely
>> >> +        * to make any progress.  To prevent a livelock, don't retry.
>> >> +        */
>> >> +       if (!(gfp_allowed_mask & __GFP_FS))
>> >> +               return 0;
>> >> +
>> >> +       /*
>> >>          * In this implementation, order <= PAGE_ALLOC_COSTLY_ORDER
>> >>          * means __GFP_NOFAIL, but that may not be true in other
>> >>          * implementations.
>> >
>> > Eek, this is precisely what we don't want and is functionally the same as
>> > what you initially proposed except it doesn't care about __GFP_NOFAIL.
>>
>> This is checking against gfp_allowed_mask, not gfp_mask.
>>
>
> gfp_allowed_mask is initialized to GFP_BOOT_MASK to start so that __GFP_FS
> is never allowed before the slab allocator is completely initialized, so
> you've now implicitly made all early boot allocations to be __GFP_NORETRY
> even though they may not pass it.

Only before interrupts are enabled, and then isn't it vulnerable to
the same livelock?  Interrupts are off, single cpu, kswapd can't run.
If an allocation ever failed, which seems unlikely, why would retrying
help?
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