From mboxrd@z Thu Jan  1 00:00:00 1970
From: Nikita Danilov <danilov@gmail.com>
Subject: Re: [PATCH 30/45] vmscan: lumpy pageout
Date: Wed, 7 Oct 2009 14:38:37 +0400
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Cc: Andrew Morton <akpm@linux-foundation.org>,
	Theodore Tso <tytso@mit.edu>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Chris Mason <chris.mason@oracle.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Li Shaohua <shaohua.li@intel.com>,
	Myklebust Trond <Trond.Myklebust@netapp.com>,
	"jens.axboe@oracle.com" <jens.axboe@oracle.com>,
	Jan Kara <jack@suse.cz>, Nick Piggin <npiggin@suse.de>,
	linux-fsdevel@vger.kernel.org
To: Wu Fengguang <fengguang.wu@intel.com>
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Hello,

 > When pageout a dirty page, try to piggy back more consecutive dirty
 > pages (up to 512KB) to improve IO efficiency.
 >
 > Only ext3/reiserfs which don't have its own aops->writepages are
 > supported in this initial version.
 >

[...]

 >  		/*
 > +		 * only write_cache_pages() supports for_reclaim for now
 > +		 */
 > +		if (!mapping->a_ops->writepages) {
 > +			wbc.range_start = (page->index + 1) << PAGE_CACHE_SHIFT;
 > +			wbc.nr_to_write = LUMPY_PAGEOUT_PAGES - 1;
 > +			generic_writepages(mapping, &wbc);
 > +		}

This might end up calling ->writepage() on a page_mapped() page,
which used to be a problem, at least for shmem (see
BUG_ON(page_mapped(page)) in shmem_writepage()).

I tried to do a similar thing in the past, but eventually decided
do explicitly call ->writepage() in a loop from vmscan.c. This
way one can also filter out referenced and active pages. See
http://linuxhacker.ru/~nikita/patches/2.6.14-rc5/05-cluster-pageout.patch,
it also contains a comment explaining shmem problem.

Thank you,
Nikita.