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    SubjectRe: [PATCH] x86: fix PAE pmd_bad bootup warning
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    On Thu, 2008-05-08 at 18:19 +0200, Hans Rosenfeld wrote:
    > On Thu, May 08, 2008 at 08:51:22AM -0700, Dave Hansen wrote:
    > > Is there anything in your dmesg?
    >
    > mm/memory.c:127: bad pmd ffff810076801040(80000000720000e7).
    >
    > > There was a discussion on LKML in the last couple of days about
    > > pmd_bad() triggering on huge pages. Perhaps we're clearing the mapping
    > > with the pmd_none_or_clear_bad(), and *THAT* is leaking the page.
    >
    > That makes sense. I remember that explicitly munmapping the huge page
    > would still work, but it doesn't. I don't quite remember what I did back
    > then to test this, but I probably made some mistake there that led me to
    > some false conclusions.

    I can't see how it would possibly work with the code that we have today,
    so I guess it was just a false assumption.

    static inline int pmd_none_or_clear_bad(pmd_t *pmd)
    {
    if (pmd_none(*pmd))
    return 1;
    if (unlikely(pmd_bad(*pmd))) {
    pmd_clear_bad(pmd);
    return 1;
    }
    return 0;
    }

    void pmd_clear_bad(pmd_t *pmd)
    {
    pmd_ERROR(*pmd);
    pmd_clear(pmd);
    }

    That pmd_clear() will simply zero out the pmd and leak the page.

    Sounds like Linus had the right idea:

    > I'd much rather have pdm_bad() etc fixed up instead, so that they do a
    > more proper test (not thinking that a PSE page is bad, since it clearly
    > isn't). And then, make them dependent on DEBUG_VM, because doing the
    > proper test will be more expensive.

    -- Dave



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