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    Subject[PATCH 05/21] mm: anon is already wrprotected
    do_anonymous_page's pte_wrprotect causes some confusion: in such a case,
    vm_page_prot must already be forcing COW, so must omit write permission,
    and so the pte_wrprotect is redundant. Replace it by a comment to that
    effect, and reword the comment on unuse_pte which also caused confusion.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>
    ---

    mm/memory.c | 7 ++++---
    mm/swapfile.c | 7 +++----
    2 files changed, 7 insertions(+), 7 deletions(-)

    --- mm04/mm/memory.c 2005-09-24 19:27:05.000000000 +0100
    +++ mm05/mm/memory.c 2005-09-24 19:27:19.000000000 +0100
    @@ -1768,13 +1768,14 @@ do_anonymous_page(struct mm_struct *mm,
    unsigned long addr)
    {
    pte_t entry;
    - struct page * page = ZERO_PAGE(addr);

    - /* Read-only mapping of ZERO_PAGE. */
    - entry = pte_wrprotect(mk_pte(ZERO_PAGE(addr), vma->vm_page_prot));
    + /* Mapping of ZERO_PAGE - vm_page_prot is readonly */
    + entry = mk_pte(ZERO_PAGE(addr), vma->vm_page_prot);

    /* ..except if it's a write access */
    if (write_access) {
    + struct page *page;
    +
    /* Allocate our own private page. */
    pte_unmap(page_table);
    spin_unlock(&mm->page_table_lock);
    --- mm04/mm/swapfile.c 2005-09-22 12:32:04.000000000 +0100
    +++ mm05/mm/swapfile.c 2005-09-24 19:27:19.000000000 +0100
    @@ -396,10 +396,9 @@ void free_swap_and_cache(swp_entry_t ent
    }

    /*
    - * Always set the resulting pte to be nowrite (the same as COW pages
    - * after one process has exited). We don't know just how many PTEs will
    - * share this swap entry, so be cautious and let do_wp_page work out
    - * what to do if a write is requested later.
    + * No need to decide whether this PTE shares the swap entry with others,
    + * just let do_wp_page work it out if a write is requested later - to
    + * force COW, vm_page_prot omits write permission from any private vma.
    *
    * vma->vm_mm->page_table_lock is held.
    */
    -
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