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

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Date:	Tue, 6 Jul 2010 21:14:28 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Mel Gorman <mel@....ul.ie>, 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 10:27:58PM +0200, Johannes Weiner wrote:
> 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)

Exactly.  Note that while the stack problems for swap writeout aren't
as bad as for filesystems as the whole allocator / extent map footprint
is missing it might still be an issue.  We still splice the whole block
I/O stack footprint over a random stack that might be filled up a lot.
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