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

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Date:	Wed, 26 Oct 2011 00:22:14 -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 Wed, Oct 26, 2011 at 12:10 AM, David Rientjes <rientjes@...gle.com> wrote:
> On Tue, 25 Oct 2011, Colin Cross wrote:
>
>> > 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?
>>
>
> If you want to claim gfp_allowed_mask as a pm-only entity, then I see no
> problem with this approach.  However, if gfp_allowed_mask would be allowed
> to temporarily change after init for another purpose then it would make
> sense to retry because another allocation with __GFP_FS on another cpu or
> kswapd could start making progress could allow for future memory freeing.
>
> The suggestion to add a hook directly into a pm-interface was so that we
> could isolate it only to suspend and, to me, is the most maintainable
> solution.
>

pm_restrict_gfp_mask seems to claim gfp_allowed_mask as owned by pm at runtime:
"gfp_allowed_mask also should only be modified with pm_mutex held,
unless the suspend/hibernate code is guaranteed not to run in parallel
with that modification"

I think we've wrapped around to Mel's original patch, which adds a
pm_suspending() helper that is implemented next to
pm_restrict_gfp_mask.  His patch puts the check inside
!did_some_progress instead of should_alloc_retry, which I prefer as it
at least keeps trying until reclaim isn't working.  Pekka was trying
to avoid adding pm-specific checks into the allocator, which is why I
stuck to the symptom (__GFP_FS is clear) rather than the cause (PM).
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