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    Subject[patch 11/14] remap_file_pages protection support: pte_present should not trigger on PTE_FILE PROTNONE ptes
    From: Paolo 'Blaisorblade' Giarrusso <>

    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

    zap_pte_range, when acting on such a pte, calls vm_normal_page and gets
    &mem_map[0], does page_remove_rmap, and we're easily in trouble, because it
    happens to find a page with mapcount == 0. And it BUGs on this!

    I've seen this trigger easily and repeatably on UML on 2.6.16-rc3. This was
    likely avoided in the past by the PageReserved test - page 0 *had* to be
    reserved on i386 (dunno on UML).

    Implementation follows for UML and i386.

    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.

    Signed-off-by: Paolo 'Blaisorblade' Giarrusso <>
    Index: linux-2.6.git/include/asm-um/pgtable.h
    --- linux-2.6.git.orig/include/asm-um/pgtable.h
    +++ linux-2.6.git/include/asm-um/pgtable.h
    @@ -158,7 +158,7 @@ extern unsigned long pg0[1024];
    #define mk_phys(a, r) ((a) + (((unsigned long) r) << REGION_SHIFT))
    #define phys_addr(p) ((p) & ~REGION_MASK)

    -#define pte_present(x) pte_get_bits(x, (_PAGE_PRESENT | _PAGE_PROTNONE))
    +#define pte_present(x) (pte_get_bits(x, (_PAGE_PRESENT)) || (pte_get_bits(x, (_PAGE_PROTNONE)) && !pte_file(x)))

    * =================================
    Index: linux-2.6.git/mm/memory.c
    --- linux-2.6.git.orig/mm/memory.c
    +++ linux-2.6.git/mm/memory.c
    @@ -624,6 +624,8 @@ static unsigned long zap_pte_range(struc

    (*zap_work) -= PAGE_SIZE;

    + /* XXX: This can trigger even if the PTE is only a PROTNONE
    + * PTE_FILE pte - we'll then extract page 0 and unmap it! */
    if (pte_present(ptent)) {
    struct page *page;

    Index: linux-2.6.git/include/asm-i386/pgtable.h
    --- linux-2.6.git.orig/include/asm-i386/pgtable.h
    +++ linux-2.6.git/include/asm-i386/pgtable.h
    @@ -204,6 +204,8 @@ extern unsigned long long __PAGE_KERNEL,
    extern unsigned long pg0[];

    #define pte_present(x) ((x).pte_low & (_PAGE_PRESENT | _PAGE_PROTNONE))
    +#define pte_present(x) (((x).pte_low & _PAGE_PRESENT) || \
    + (((x).pte_low & (_PAGE_PROTNONE|_PAGE_FILE)) == _PAGE_PROTNONE))
    #define pte_clear(mm,addr,xp) do { set_pte_at(mm, addr, xp, __pte(0)); } while (0)

    /* To avoid harmful races, pmd_none(x) should check only the lower when PAE */
    Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
    Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
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      Last update: 2006-04-30 19:35    [W:0.021 / U:8.864 seconds]
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