linux-kernel - Re: [patch 03/13] io-accounting: write accounting

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Message-ID: <457FBDBE.10102@FreeBSD.org>
Date:	Wed, 13 Dec 2006 00:45:50 -0800
From:	Suleiman Souhlal <ssouhlal@...eBSD.org>
To:	akpm@...l.org
CC:	linux-kernel@...r.kernel.org, balbir@...ibm.com, csturtiv@....com,
	daw@....com, guillaume.thouvenin@...l.net, jlan@....com,
	nagar@...son.ibm.com, tee@....com
Subject: Re: [patch 03/13] io-accounting: write accounting

akpm@...l.org wrote:
> From: Andrew Morton <akpm@...l.org>
> 
> Accounting writes is fairly simple: whenever a process flips a page from clean
> to dirty, we accuse it of having caused a write to underlying storage of
> PAGE_CACHE_SIZE bytes.

On architectures where dirtying a page doesn't cause a page fault (like i386), couldn't you end up billing the wrong process (in fact, I think that even on other archituctures set_page_dirty() doesn't get called immediately in the page fault handler)? 

AFAICS, set_page_dirty() is mostly called when trying to unmap a page when trying to shrink LRU lists, and there is no guarantee that this happens under the process that dirtied it (in fact, the set_page_dirty() is often done by kswapd).

-- Suleiman
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