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

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Date:	Tue, 15 Nov 2011 16:28:36 -0800
From:	Colin Cross <ccross@...roid.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Tue, Nov 15, 2011 at 4:22 PM, Minchan Kim <minchan.kim@...il.com> wrote:
> On Tue, Nov 15, 2011 at 05:36:56PM +0000, Mel Gorman wrote:
>> On Wed, Nov 16, 2011 at 01:13:30AM +0900, Minchan Kim wrote:
>> The impact would be that during the time between processes been frozen
>> and storage being suspended, GFP_NOIO allocations that used to call
>> wait_iff_congested and retry while kswapd does its thing will return
>> failure instead. These GFP_NOIO allocations that used to succeed will
>> now fail in rare cases during suspend and I don't think we want that.
>>
>> Is this what you meant or had you something else in mind?
>>
>
> You read my mind exactly!
>
> I thought hibernation process is as follows,
>
> freeze user processes
> oom_disable
> hibernate_preallocate_memory
> freeze kernel processes(include kswapd)
> pm_restrict_gfp_mask
> swsusp_save
>
> My guessing is hibernate_prealocate_memory should reserve all memory needed
> for hibernation for reclaimaing pages of kswapd because kswapd just would be
> stopped so during swsusp_save, page reclaim should not be occured.
>
> But being see description of patch, my guess seems wrong.
> Now the problem happens and it means page reclaim happens during swsusp_save.
> Colin or someone could confirm this?

The problem I see is during suspend, not hibernation.  The particular
allocation that usually causes the problem is the pgd_alloc for page
tables when re-enabling the 2nd cpu during resume, which is odd as
those same page tables were freed during suspend.  I guess an
unfreezable kernel thread allocated that memory between the free and
re-allocation.

> If so, could we reserve more memory when we preallocate hibernation memory
> for avoiding page reclaim without kswapd?
>
> --
> Kind regards,
> Minchan Kim
>
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