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

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Message-Id: <20100616091755.7121c7d3.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 16 Jun 2010 09:17:55 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Rik van Riel <riel@...hat.com>, 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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

On Tue, 15 Jun 2010 15:16:01 +0100
Mel Gorman <mel@....ul.ie> wrote:

> 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.
> 
yes. It's only called from 
	- page fault
	- add_to_page_cache()

I think we'll see no stack problem. Now, memcg doesn't wakeup kswapd for
reclaiming memory, it needs direct writeback.

Thanks,
-Kame

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