linux-kernel - Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

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Date:	Tue, 6 Jul 2010 16:25:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Hellwig <hch@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

On Tue, Jul 06, 2010 at 08:24:57PM +0900, Minchan Kim wrote:
> Hi, Mel.
> 
> On Tue, Jul 6, 2010 at 7:12 PM, Mel Gorman <mel@....ul.ie> wrote:
> > On Tue, Jul 06, 2010 at 09:36:41AM +0900, KOSAKI Motohiro wrote:
> >> Hello,
> >>
> >> > Ok, that's reasonable as I'm still working on that patch. For example, the
> >> > patch disabled anonymous page writeback which is unnecessary as the stack
> >> > usage for anon writeback is less than file writeback.
> >>
> >> How do we examine swap-on-file?
> >>
> >
> > Anything in particular wrong with the following?
> >
> > /*
> >  * For now, only kswapd can writeback filesystem pages as otherwise
> >  * there is a stack overflow risk
> >  */
> > static inline bool reclaim_can_writeback(struct scan_control *sc,
> >                                        struct page *page)
> > {
> >        return !page_is_file_cache(page) || current_is_kswapd();
> > }
> >
> > Even if it is a swapfile, I didn't spot a case where the filesystems
> > writepage would be called. Did I miss something?
> 
> 
> As I understand Kosaki's opinion, He said that if we make swapout in
> pageout, it isn't a problem in case of swap device since swapout of
> block device is light

Sure

> but it is still problem in case of swap file.
> That's because swapout on swapfile cause file system writepage which
> makes kernel stack overflow.
> 

I don't *think* this is a problem unless I missed where writing out to
swap enters teh filesystem code. I'll double check.

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