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

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Date:	Tue, 15 Jun 2010 10:04:24 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
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 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

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