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

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Message-ID: <20100706202758.GC18210@cmpxchg.org>
Date:	Tue, 6 Jul 2010 22:27:58 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	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>,
	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 04:25:39PM +0100, Mel Gorman wrote:
> On Tue, Jul 06, 2010 at 08:24:57PM +0900, Minchan Kim wrote:
> > 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.

It bypasses the fs.  On swapon, the blocks are resolved
(mm/swapfile.c::setup_swap_extents) and then the writeout path uses
bios directly (mm/page_io.c::swap_writepage).

(GFP_NOFS still includes __GFP_IO, so allows swapping)

	Hannes
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