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    SubjectRe: [patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes
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    On Saturday 06 May 2006 12:03, Nick Piggin wrote:
    > Blaisorblade wrote:
    > > On Tuesday 02 May 2006 05:53, Nick Piggin wrote:
    > >>blaisorblade@yahoo.it wrote:
    > >>>From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
    > >>>
    > >>>pte_present(pte) implies that pte_pfn(pte) is valid. Normally even with
    > >>> a _PAGE_PROTNONE pte this holds, but not when such a PTE is installed
    > >>> by the new install_file_pte; previously it didn't store protections,
    > >>> only file offsets, with the patches it also stores protections, and can
    > >>> set _PAGE_PROTNONE|_PAGE_FILE.
    > >
    > > What could be done is to set a PTE with "no protection", use another bit
    > > rather than _PAGE_PROTNONE. This wastes one more bit but doable.

    > I see.

    > I guess your problem is that you're overloading the pte protection bits
    > for present ptes as protection bits for not present (file) ptes. I'd rather
    > you just used a different encoding for file pte protections then.

    Yes, this is what I said above, so we agree; and indeed this overloading was
    decided when the present problem didn't trigger, so it can now change. As
    detailed in the patch description, the previous PageReserved handling
    prevented freeing page 0 and hided this.

    > "Wasting" a bit seems much more preferable for this very uncommon case (for
    > most people) rather than bloating pte_present check, which is called in
    > practically every performance critical inner loop).

    Yes, I thought about this problem, I wasn't sure how hard it was.

    > That said, if the patch is i386/uml specific then I don't have much say in
    > it.

    It's presently specific, but will probably extend. Implementations for some
    other archs were already sent and I've collected them (will send
    afterwards,I've avoided excess bloat).

    > If Ingo/Linus and Jeff/Yourself, respectively, accept the patch, then
    > fine.

    > But I think you should drop the comment from the core code. It seems wrong.

    Yep, forgot there, thanks for reminding, I've now removed it.
    --
    Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
    Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
    http://www.user-mode-linux.org/~blaisorblade

    Chiacchiera con i tuoi amici in tempo reale!
    http://it.yahoo.com/mail_it/foot/*http://it.messenger.yahoo.com

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