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From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 5/6] writeback: try more writeback as long as something
 was written
Date: Thu, 21 Apr 2011 00:39:40 -0400
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Cc: Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
	Mel Gorman <mel@linux.vnet.ibm.com>,
	Dave Chinner <david@fromorbit.com>,
	Trond Myklebust <Trond.Myklebust@netapp.com>,
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To: Wu Fengguang <fengguang.wu@intel.com>
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On Thu, Apr 21, 2011 at 11:33:25AM +0800, Wu Fengguang wrote:
> I collected the writeback_single_inode() traces (patch attached for
> your reference) each for several test runs, and find much more
> I_DIRTY_PAGES after patchset. Dave, do you know why there are so many
> I_DIRTY_PAGES (or radix tag) remained after the XFS ->writepages() call,
> even for small files?

What is your defintion of a small file?  As soon as it has multiple
extents or holes there's absolutely no way to clean it with a single
writepage call.  Also XFS tries to operate as non-blocking as possible
if the non-blocking flag is set in the wbc, but that flag actually
seems to be dead these days.