linux-kernel - Re: [patch 8/6] mm: fix cpdfio vs fault race

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Date:	Wed, 7 Mar 2007 03:34:00 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>, miklos@...redi.hu,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 8/6] mm: fix cpdfio vs fault race

On Wed, 7 Mar 2007 03:20:38 -0800 Andrew Morton <akpm@...ux-foundation.org> wrote:

> 
> > ===================================================================
> > --- linux-2.6.orig/mm/memory.c
> > +++ linux-2.6/mm/memory.c
> > @@ -1676,6 +1676,17 @@ gotten:
> >  unlock:
> >  	pte_unmap_unlock(page_table, ptl);
> >  	if (dirty_page) {
> > +		/*
> > +		 * Yes, Virginia, this is actually required to prevent a race
> > +		 * with clear_page_dirty_for_io() from clearing the page dirty
> > +		 * bit after it clear all dirty ptes, but before a racing
> > +		 * do_wp_page installs a dirty pte.
> > +		 *
> > +		 * do_fault is protected similarly by holding the page lock
> > +		 * after the dirty pte is installed.
> > +		 */
> > +		lock_page(dirty_page);
> > +		unlock_page(dirty_page);
> >  		set_page_dirty_balance(dirty_page);
> >  		put_page(dirty_page);
> 
> Yes, I think that'll plug it.  A wait_on_page_locked() should suffice.

Or will it?  Suppose after the unlock_page() a _second_
clear_page_dirty_for_io() gets run - the same thing happens?

Extending the lock_page() coverage around the set_page_dirty() would
prevent that.

I guess not needed - the second clear_page_dirty_for_io() will have cleaned the
pte.
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