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    Subject[PATCH 14/25] mm: simplify get_user_pages() PTE bit handling
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    From: Dave Hansen <dave.hansen@linux.intel.com>

    The current get_user_pages() code is a wee bit more complicated
    than it needs to be for pte bit checking. Currently, it establishes
    a mask of required pte _PAGE_* bits and ensures that the pte it
    goes after has all those bits.

    We need to use the bits for our _PAGE_PRESENT check since
    pte_present() is also true for _PAGE_PROTNONE, and we have no
    accessor for _PAGE_USER, so need it there as well.

    But we might as well just use pte_write() since we have it and
    let the compiler work its magic on optimizing it.

    This also consolidates the three identical copies of this code.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    ---

    b/arch/x86/mm/gup.c | 34 +++++++++++++++++-----------------
    1 file changed, 17 insertions(+), 17 deletions(-)

    diff -puN arch/x86/mm/gup.c~pkeys-16-gup-swizzle arch/x86/mm/gup.c
    --- a/arch/x86/mm/gup.c~pkeys-16-gup-swizzle 2015-09-28 11:39:47.203239951 -0700
    +++ b/arch/x86/mm/gup.c 2015-09-28 11:39:47.206240088 -0700
    @@ -63,6 +63,19 @@ retry:
    #endif
    }

    +static inline int pte_allows_gup(pte_t pte, int write)
    +{
    + /*
    + * Note that pte_present() is true for !_PAGE_PRESENT
    + * but _PAGE_PROTNONE, so we can not use it here.
    + */
    + if (!(pte_flags(pte) & (_PAGE_PRESENT|_PAGE_USER)))
    + return 0;
    + if (write && !pte_write(pte))
    + return 0;
    + return 1;
    +}
    +
    /*
    * The performance critical leaf functions are made noinline otherwise gcc
    * inlines everything into a single function which results in too much
    @@ -71,13 +84,8 @@ retry:
    static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
    unsigned long end, int write, struct page **pages, int *nr)
    {
    - unsigned long mask;
    pte_t *ptep;

    - mask = _PAGE_PRESENT|_PAGE_USER;
    - if (write)
    - mask |= _PAGE_RW;
    -
    ptep = pte_offset_map(&pmd, addr);
    do {
    pte_t pte = gup_get_pte(ptep);
    @@ -88,8 +96,8 @@ static noinline int gup_pte_range(pmd_t
    pte_unmap(ptep);
    return 0;
    }
    -
    - if ((pte_flags(pte) & (mask | _PAGE_SPECIAL)) != mask) {
    + if (!pte_allows_gup(pte, write) ||
    + pte_special(pte)) {
    pte_unmap(ptep);
    return 0;
    }
    @@ -117,15 +125,11 @@ static inline void get_head_page_multipl
    static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
    unsigned long end, int write, struct page **pages, int *nr)
    {
    - unsigned long mask;
    pte_t pte = *(pte_t *)&pmd;
    struct page *head, *page;
    int refs;

    - mask = _PAGE_PRESENT|_PAGE_USER;
    - if (write)
    - mask |= _PAGE_RW;
    - if ((pte_flags(pte) & mask) != mask)
    + if (!pte_allows_gup(pte, write))
    return 0;
    /* hugepages are never "special" */
    VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
    @@ -194,15 +198,11 @@ static int gup_pmd_range(pud_t pud, unsi
    static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
    unsigned long end, int write, struct page **pages, int *nr)
    {
    - unsigned long mask;
    pte_t pte = *(pte_t *)&pud;
    struct page *head, *page;
    int refs;

    - mask = _PAGE_PRESENT|_PAGE_USER;
    - if (write)
    - mask |= _PAGE_RW;
    - if ((pte_flags(pte) & mask) != mask)
    + if (!pte_allows_gup(pte, write))
    return 0;
    /* hugepages are never "special" */
    VM_BUG_ON(pte_flags(pte) & _PAGE_SPECIAL);
    _

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