Re: [PATCH] mm: fix page_mkclean_one

From: Linus Torvalds
Date: Thu Dec 28 2006 - 12:17:29 EST

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..

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