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

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Message-ID: <457FD777.9040703@FreeBSD.org>
Date:	Wed, 13 Dec 2006 02:35:35 -0800
From:	Suleiman Souhlal <ssouhlal@...eBSD.org>
To:	Andrew Morton <akpm@...l.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

Andrew Morton wrote:
> On Wed, 13 Dec 2006 00:45:50 -0800
> Suleiman Souhlal <ssouhlal@...eBSD.org> wrote:
> 
> 
>>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)? 
> 
> 
> Yes, that would be a problem in 2.6.18 and earlier.
> 
> In 2.6.19 and later, we do take a fault when transitioning a page from
> pte-clean to pte-dirty.  That was done to get the dirty-page accounting
> right - to avoid the all-of-memory-is-dirty-but-the-kernel-doesn't-know-it
> problem.

Ah yes indeed. I'm unable to keep up with all the new developments. :-(

However, if my understanding of this code is correct, it seems that the
page fault is only done for shared writable VMAs, so you still can't
rely on set_page_dirty() always being called by the process that
dirtied the page in the first place.

Am I wrong?

-- Suleiman
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