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

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Message-ID: <20100615141601.GL26788@csn.ul.ie>
Date:	Tue, 15 Jun 2010 15:16:01 +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 10:04:24AM -0400, Rik van Riel wrote:
> On 06/15/2010 09:59 AM, Mel Gorman wrote:
>> On Tue, Jun 15, 2010 at 09:34:18AM -0400, Rik van Riel wrote:
>>> On 06/15/2010 07:45 AM, Mel Gorman wrote:
>
>>>>>>
>>>>>> +/* 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;
>>>>>> +}
>
>>> 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?
>
> mem_cgroup_hierarchical_reclaim -> try_to_free_mem_cgroup_pages
>

But in turn, where is mem_cgroup_hierarchical_reclaim called from direct
reclaim? It appears to be only called from the fault path or as a result
of the memcg changing size.

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