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

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Date:	Wed, 13 Dec 2006 00:59:54 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Suleiman Souhlal <ssouhlal@...eBSD.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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

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.


> 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).

hm, that code is still there in zap_pte_range().  If all is well, that
set_page_dirty() call should never return true.  Peter did, you ever test
for that?

(Well, it might return true in rare races, because zap_pte_range() doesn't
lock the pages)
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