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    SubjectRe: pgd_none_or_clear_bad strangeness?
    On Wed, Oct 03, 2007 at 07:18:23PM +0100, Hugh Dickins wrote:
    > On Wed, 3 Oct 2007, Nick Piggin wrote:
    > > On Tue, Oct 02, 2007 at 05:20:03PM -0500, Matt Mackall wrote:
    > > > In lib/pagewalk.c, I've been using the various forms of
    > > > {pgd,pud,pmd}_none_or_clear_bad while walking page tables as that
    > > > seemed the canonical way to do things. Lately (eg with -rc7-mm1),
    > > > these have been triggering messages like "bad pgd 0x01e3" and causing
    > > > nasty double faults. It appears this is actually triggered at the pmd
    > > > level (mm/memory.c:116), though it appears to produce the wrong
    > > > message.
    >
    > I guess the "wrong message" is an artifact of pud/pmd folding;
    > but I get too confused by the different levels myself to want to
    > think more about it - I'll just assume it's "right" somehow ;)
    >
    > > >
    > > > Has something changed here? I'm pretty sure this used to work! Is this
    >
    > I don't know of anything changing here, sorry.
    >
    > > > not a kosher thing to do? Does it make any sense I'd repeatedly run
    > > > into a bad pmd in the middle of bash's page table right after boot?
    > > > The simple _none variant seems to work, but I worry that it's papering
    > > > over a real problem.
    > >
    > > No, I think that should be the right thing to do for userspace pages.
    > > You're not walking into a hugetlb area or a kernel mapping are you?
    > > (the bad pgd: line could be important... 0x01e3 would be a linear kernel
    > > mapping I think?).
    >
    > I should have spent more time reading Nick's reply and less time trying
    > to work it out for myself! Yes, that's the conclusion I came to, for
    > some reason you're now going beyond the user vmas and walking into the
    > linear kernel mapping, which has _PAGE_GLOBAL and _PAGE_PSE bits set.

    Indeed, that's precisely what's happening. I'm walking one page past
    the end of userspace.

    And the reason is I changed my walker from using for loops to do/while
    loops at Nick's insistance, so start==end no longer gets noticed
    immediately. This also explains why the bug doesn't manifest in
    lguest: no PSE mappings.

    Thanks, guys!

    --
    Mathematics is the supreme nostalgia of our time.
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