linux-kernel - Re: [RFC PATCH 2/2] Don't continue reclaim if the system have plenty free memory

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Date:	Mon, 13 Jul 2009 09:37:50 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [RFC PATCH 2/2] Don't continue reclaim if the system have plenty  free memory

> On Thu, Jul 9, 2009 at 2:08 PM, KOSAKI
> Motohiro<kosaki.motohiro@...fujitsu.com> wrote:
> >> Hi, Kosaki.
> >>
> >> On Tue, Jul 7, 2009 at 6:48 PM, KOSAKI
> >> Motohiro<kosaki.motohiro@...fujitsu.com> wrote:
> >> > Subject: [PATCH] Don't continue reclaim if the system have plenty free memory
> >> >
> >> > On concurrent reclaim situation, if one reclaimer makes OOM, maybe other
> >> > reclaimer can stop reclaim because OOM killer makes enough free memory.
> >> >
> >> > But current kernel doesn't have its logic. Then, we can face following accidental
> >> > 2nd OOM scenario.
> >> >
> >> > 1. System memory is used by only one big process.
> >> > 2. memory shortage occur and concurrent reclaim start.
> >> > 3. One reclaimer makes OOM and OOM killer kill above big process.
> >> > 4. Almost reclaimable page will be freed.
> >> > 5. Another reclaimer can't find any reclaimable page because those pages are
> >> > ? already freed.
> >> > 6. Then, system makes accidental and unnecessary 2nd OOM killer.
> >> >
> >>
> >> Did you see the this situation ?
> >> Why I ask is that we have already a routine for preventing parallel
> >> OOM killing in __alloc_pages_may_oom.
> >>
> >> Couldn't it protect your scenario ?
> >
> > Can you please see actual code of this patch?
> 
> I mean follow as,
> 
> static inline struct page *
> __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>         struct zonelist *zonelist, enum zone_type high_zoneidx,
> ...
> <snip>
> 
>         /*
>          * Go through the zonelist yet one more time, keep very high watermark
>          * here, this is only to catch a parallel oom killing, we must fail if
>          * we're still under heavy pressure.
>          */
>         page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask,
>                 order, zonelist, high_zoneidx,
>                 ALLOC_WMARK_HIGH|ALLOC_CPUSET,
>                 preferred_zone, migratetype);

Thanks, I catch your point.
Yes, the issue explained my description only happen old distro kernel.
I haven't notice this issue was already fixed. very thanks.

but above fix doesn't make sense. it mean
 - concurrent reclaim can drop too many usable memory
 - but only gurantee it doesn't cause oom

Then, I'll fix my patch description.



> > Those two patches fix different problem.
> >
> > 1/2 fixes the issue of that concurrent direct reclaimer makes
> > too many isolated pages.
> > 2/2 fixes the issue of that reclaim and exit race makes accidental oom.
> >
> >
> >> If it can't, Could you explain the scenario in more detail ?
> >
> > __alloc_pages_may_oom() check don't effect the threads of already
> > entered reclaim. it's obvious.
> 
> Threads which are entered into direct reclaim mode will call
> __alloc_pages_may_oom before out_of_memory.
> At that time, if one big process is killed a while ago,
> get_page_from_freelist in __alloc_pages_may_oom will be succeeded at
> last. So I think it doesn't occur OOM.
> 
> But in that case, we suffered from unnecessary page scanning per each
> priority(12~0). So in this case, your patch is good to me. then you
> would be better to change log. :)






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