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    SubjectRe: [patch] do_no_pfn()
    On 20 Sep 2006 03:25:25 -0400
    Jes Sorensen <> wrote:

    > Implement do_no_pfn() for handling mapping of memory without a struct
    > page backing it. This avoids creating fake page table entries for
    > regions which are not backed by real memory.
    > This feature is used by the MSPEC driver and other users, where it is
    > highly undesirable to have a struct page sitting behind the page
    > (for instance if the page is accessed in cached mode via the struct
    > page in parallel to the the driver accessing it uncached, which can
    > result in data corruption on some architectures, such as ia64).
    > This version uses specific NOPFN_{SIGBUS,OOM} return values, rather
    > than expect all negative pfn values would be an error. It also bugs on
    > cow mappings as this would not work with the VM.

    How does this followup look?

    We don't want the rarely-used do_no_pfn() to get inlined in the oft-used
    handle_pte_fault(), using up icache. Mark it noinline and unlikely.

    --- a/mm/memory.c~do_no_pfn-tweaks
    +++ a/mm/memory.c
    @@ -2276,8 +2276,10 @@ oom:
    * It is expected that the ->nopfn handler always returns the same pfn
    * for a given virtual mapping.
    + *
    + * Mark this `noinline' to prevent it from bloating the main pagefault code.
    -static int do_no_pfn(struct mm_struct *mm, struct vm_area_struct *vma,
    +static noinline int do_no_pfn(struct mm_struct *mm, struct vm_area_struct *vma,
    unsigned long address, pte_t *page_table, pmd_t *pmd,
    int write_access)
    @@ -2376,7 +2378,7 @@ static inline int handle_pte_fault(struc
    return do_no_page(mm, vma, address,
    pte, pmd,
    - if (vma->vm_ops->nopfn)
    + if (unlikely(vma->vm_ops->nopfn))
    return do_no_pfn(mm, vma, address, pte,
    pmd, write_access);
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      Last update: 2006-09-22 21:53    [W:0.021 / U:45.140 seconds]
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