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


    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.

    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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