linux-kernel - Re: [patch 10/21] buffer heads: Support slab defrag

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Message-ID: <20080520233015.GY103491721@sgi.com>
Date:	Wed, 21 May 2008 09:30:15 +1000
From:	David Chinner <dgc@....com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	David Chinner <dgc@....com>, Christoph Lameter <clameter@....com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	andi@...stfloor.org, Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>, mpm@...enic.com
Subject: Re: [patch 10/21] buffer heads: Support slab defrag

On Wed, May 21, 2008 at 03:22:56AM +0400, Evgeniy Polyakov wrote:
> On Wed, May 21, 2008 at 02:25:05AM +0400, Evgeniy Polyakov (johnpol@....mipt.ru) wrote:
> > > Oh, god no. Let's not put the inode_lock right at the top of
> > > the VM page cleaning path. We don't need to modify inode state,
> > > the superblock dirty lists, etc - all we need to do is write
> > > dirty pages on a given mapping in a more efficient manner.
> > 
> > I'm not advocating that, but having swap on reclaim does not hurt
> > anyone, this is essentially the same, but with different underlying
> > storage. System will do that anyway sooner or later during usual
> > writeback, which in turn can be a result of the same reclaim...
> 
> And actually having tiny operations under inode_lock is the last thing
> to worry about when we are about to start writing pages to disk because
> memory is so fragmented that we need to move things around.
> 
> That is the simplest from the typing viewpoint, one can also do
> something like that:
> 
> struct address_space *mapping = page->mapping;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> struct writeback_control wbc = {
> 	.bdi = bdi,
> 	.sync_mode = WB_SYNC_ALL, /* likly we want to wait... */
> 	.older_than_this = NULL,
> 	.nr_to_write = 13,
> 	.range_cyclic = 0,
> 	.range_start = start_index,
> 	.range_end = end_index
> };
> 
> do_writepages(mapping, &wbc);

Which is the exact implementation of

	filemap_fdatawrite_range(mapping, start, end);

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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