linux-kernel - Re: [PATCH] mm: fix page_mkclean

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Date:	Thu, 28 Dec 2006 09:15:39 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
cc:	David Miller <davem@...emloft.net>, ranma@...edrich.de,
	gordonfarquharson@...il.com, tbm@...ius.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, andrei.popa@...eo.ro,
	Andrew Morton <akpm@...l.org>, hugh@...itas.com,
	nickpiggin@...oo.com.au, arjan@...radead.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix page_mkclean_one



On Thu, 28 Dec 2006, Zhang, Yanmin wrote:
> 
> The test program is a process to write/read data. pdflush might write data
> to disk asynchronously. After pdflush writes a page to disk, it will call (either by
> softirq) clear_page_dirty to clear the dirty bit after getting the interrupt
> notification.

That would indeed be a horrible bug. However, we don't do 
"clear_page_dirty()" _after_ the IO has completed, we do it _before_ the 
IO starts.

If you can actually find a place that does clear_page_dirty _after_ IO, 
then yes, you've just found the bug. But I haven't found it.

So the rule is _always_:

 - call "clear_page_dirty_for_io()" with the page lock held, and _before_ 
   the IO starts.
 - do "set_page_writeback()" before unlocking the page again
 - do a "end_page_writeback()" when the IO actually finishes.

and any code sequence that doesn't honor those rules would be buggy. A 
beer for anybody that finds it..

		Linus
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