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    SubjectRe: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)
    FromAndrei Popa <>
    DateFri, 29 Dec 2006 13:16:46 +0200
    On Fri, 2006-12-29 at 02:48 -0800, Linus Torvalds wrote:
    > 
    > On Fri, 29 Dec 2006, Linus Torvalds wrote:
    > > 
    > > Hmm? I'd love it if somebody else wrote the patch and tested it, because 
    > > I'm getting sick and tired of this bug ;)
    > 
    > Who the hell am I kidding? I haven't been able to sleep right for the last 
    > few days over this bug. It was really getting to me.
    > 
    > And putting on the thinking cap, there's actually a fairly simple an 
    > nonintrusive patch. It still has a tiny tiny race (see the comment), but I 
    > bet nobody can really hit it in real life anyway, and I know several ways 
    > to fix it, so I'm not really _that_ worried about it.
    > 
    > The patch is mostly a comment. The "real" meat of it is actually just a 
    > few lines.
    > 
    > Can anybody get corruption with this thing applied? It goes on top of 
    > plain v2.6.20-rc2.
    
    Tested with rtorrent and there is no corruption.
    
    
    > 
    > 		Linus
    > 
    > ----
    > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    > index b3a198c..ec01da1 100644
    > --- a/mm/page-writeback.c
    > +++ b/mm/page-writeback.c
    > @@ -862,17 +862,46 @@ int clear_page_dirty_for_io(struct page *page)
    >  {
    >  	struct address_space *mapping = page_mapping(page);
    > 
    > -	if (!mapping)
    > -		return TestClearPageDirty(page);
    > -
    > -	if (TestClearPageDirty(page)) {
    > -		if (mapping_cap_account_dirty(mapping)) {
    > -			page_mkclean(page);
    > +	if (mapping && mapping_cap_account_dirty(mapping)) {
    > +		/*
    > +		 * Yes, Virginia, this is indeed insane.
    > +		 *
    > +		 * We use this sequence to make sure that
    > +		 *  (a) we account for dirty stats properly
    > +		 *  (b) we tell the low-level filesystem to
    > +		 *      mark the whole page dirty if it was
    > +		 *      dirty in a pagetable. Only to then
    > +		 *  (c) clean the page again and return 1 to
    > +		 *      cause the writeback.
    > +		 *
    > +		 * This way we avoid all nasty races with the
    > +		 * dirty bit in multiple places and clearing
    > +		 * them concurrently from different threads.
    > +		 *
    > +		 * Note! Normally the "set_page_dirty(page)"
    > +		 * has no effect on the actual dirty bit - since
    > +		 * that will already usually be set. But we
    > +		 * need the side effects, and it can help us
    > +		 * avoid races.
    > +		 *
    > +		 * We basically use the page "master dirty bit"
    > +		 * as a serialization point for all the different
    > +		 * threds doing their things.
    > +		 *
    > +		 * FIXME! We still have a race here: if somebody
    > +		 * adds the page back to the page tables in
    > +		 * between the "page_mkclean()" and the "TestClearPageDirty()",
    > +		 * we might have it mapped without the dirty bit set.
    > +		 */
    > +		if (page_mkclean(page))
    > +			set_page_dirty(page);
    > +		if (TestClearPageDirty(page)) {
    >  			dec_zone_page_state(page, NR_FILE_DIRTY);
    > +			return 1;
    >  		}
    > -		return 1;
    > +		return 0;
    >  	}
    > -	return 0;
    > +	return TestClearPageDirty(page);
    >  }
    >  EXPORT_SYMBOL(clear_page_dirty_for_io);
    > 
    
    -
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