linux-kernel - Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

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Message-ID: <20100615135928.GK26788@csn.ul.ie>
Date:	Tue, 15 Jun 2010 14:59:28 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

On Tue, Jun 15, 2010 at 09:34:18AM -0400, Rik van Riel wrote:
> On 06/15/2010 07:45 AM, Mel Gorman wrote:
>> On Mon, Jun 14, 2010 at 05:55:51PM -0400, Rik van Riel wrote:
>>> On 06/14/2010 07:17 AM, Mel Gorman wrote:
>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 4856a2a..574e816 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -372,6 +372,12 @@ int write_reclaim_page(struct page *page, struct address_space *mapping,
>>>>    	return PAGE_SUCCESS;
>>>>    }
>>>>
>>>> +/* kswapd and memcg can writeback as they are unlikely to overflow stack */
>>>> +static inline bool reclaim_can_writeback(struct scan_control *sc)
>>>> +{
>>>> +	return current_is_kswapd() || sc->mem_cgroup != NULL;
>>>> +}
>>>> +
>>>
>>> I'm not entirely convinced on this bit, but am willing to
>>> be convinced by the data.
>>>
>>
>> Which bit?
>>
>> You're not convinced that kswapd should be allowed to write back?
>> You're not convinced that memcg should be allowed to write back?
>> You're not convinced that direct reclaim writing back pages can overflow
>> 	the stack?
>
> If direct reclaim can overflow the stack, so can direct
> memcg reclaim.  That means this patch does not solve the
> stack overflow, while admitting that we do need the
> ability to get specific pages flushed to disk from the
> pageout code.
>

What path is taken with memcg != NULL that could overflow the stack? I
couldn't spot one but mm/memcontrol.c is a bit tangled so finding all
its use cases is tricky. The critical path I had in mind though was
direct reclaim and for that path, memcg == NULL or did I miss something?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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