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    SubjectRe: [patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes
    Date
    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.

    > Why is this combination useful? Can't you just drop the _PAGE_FILE from
    > _PAGE_PROTNONE ptes?

    I must think on this, but the semantics are not entirely the same between the
    two cases. You have no page attached when _PAGE_FILE is there, but a page is
    attached to the PTE with only _PAGE_PROTNONE. Testing that via VM_MANYPROTS
    is just as slow as-is (can be changed with code duplication for the linear
    and non-linear cases).

    The application semantics can also be different when you remap as read/write
    that page - the app could have stored an offset there (this is less definite
    since you can't remap & keep the offset currently).

    Also, this wouldn't solve the problem, it would make the solution harder: how
    do I know that there's no page to call page_remove_rmap() on, without
    _PAGE_FILE?

    I thought to change _PAGE_PROTNONE: it is used to hold a page present and
    referenced but unaccessible. It seems it could be released when
    _PAGE_PROTNONE is set, but for anonymous memory it's impossible. When I've
    asked Hugh about this, he imagined the case when an application faults in a
    page in a VMA then mprotects(PROT_NONE) it; the PTE is set as PROT_NONE. We
    can avoid that in the VM_MAYSHARE case (VM_SHARED or PROT_SHARED was set but
    the file is readonly), but not when anonymous memory is present - the
    application could want it back.

    > > To avoid additional overhead, I also considered adding likely() for
    > > _PAGE_PRESENT and unlikely() for the rest, but I'm uncertain about
    > > validity of possible [un]likely(pte_present()) occurrences.
    >
    > Not present pages are likely to be pretty common when unmapping.

    Ok, only unlikely for test on _PAGE_PROTNONE and ! _PAGE_FILE.
    --
    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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