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    SubjectRe: Ok, explained.. (was Re: [PATCH] mm: fix page_mkclean_one)
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    Date
    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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