linux-kernel - Re: [PATCH -mm] throttle direct reclaim when too many pages are isolated already

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Date:	Wed, 15 Jul 2009 20:38:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH -mm] throttle direct reclaim when too many pages are
 isolated already

On Wed, 15 Jul 2009 23:28:14 -0400 Rik van Riel <riel@...hat.com> wrote:

> Andrew Morton wrote:
> > On Wed, 15 Jul 2009 23:10:43 -0400 Rik van Riel <riel@...hat.com> wrote:
> > 
> >> Andrew Morton wrote:
> >>> On Wed, 15 Jul 2009 22:38:53 -0400 Rik van Riel <riel@...hat.com> wrote:
> >>>
> >>>> When way too many processes go into direct reclaim, it is possible
> >>>> for all of the pages to be taken off the LRU.  One result of this
> >>>> is that the next process in the page reclaim code thinks there are
> >>>> no reclaimable pages left and triggers an out of memory kill.
> >>>>
> >>>> One solution to this problem is to never let so many processes into
> >>>> the page reclaim path that the entire LRU is emptied.  Limiting the
> >>>> system to only having half of each inactive list isolated for
> >>>> reclaim should be safe.
> >>>>
> >>> Since when?  Linux page reclaim has a bilion machine years testing and
> >>> now stuff like this turns up.  Did we break it or is this a
> >>> never-before-discovered workload?
> >> It's been there for years, in various forms.  It hardly ever
> >> shows up, but Kosaki's patch series give us a nice chance to
> >> fix it for good.
> > 
> > OK.
> > 
> >>>> @@ -1049,6 +1070,10 @@ static unsigned long shrink_inactive_lis
> >>>>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> >>>>  	int lumpy_reclaim = 0;
> >>>>  
> >>>> +	while (unlikely(too_many_isolated(zone, file))) {
> >>>> +		schedule_timeout_interruptible(HZ/10);
> >>>> +	}
> >>> This (incorrectly-laid-out) code is a no-op if signal_pending().
> >> Good point, I should add some code to break out of page reclaim
> >> if a fatal signal is pending,
> > 
> > We can't just return NULL from __alloc_pages(), and if we can't
> > get a page from the freelists then we're just going to have to keep
> > reclaiming.  So I'm not sure how we can do this.
> 
> If we are stuck at this point in the page reclaim code,
> it is because too many other tasks are reclaiming pages.
> 
> That makes it fairly safe to just return SWAP_CLUSTER_MAX
> here and hope that __alloc_pages() can get a page.
> 
> After all, if __alloc_pages() thinks it made progress,
> but still cannot make the allocation, it will call the
> pageout code again.

Which will immediately return because the caller still has
fatal_signal_pending()?

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