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    SubjectRe: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page
    On Wed, Apr 15, 2009 at 05:05:54PM +0900, KOSAKI Motohiro wrote:
    > - /*
    > - * If the page is mlock()d, we cannot swap it out.
    > - * If it's recently referenced (perhaps page_referenced
    > - * skipped over this mm) then we should reactivate it.
    > - */
    > if (!migration) {
    > + if (PageSwapCache(page) &&
    > + page_count(page) != page_mapcount(page) + 2) {
    > + ret = SWAP_FAIL;
    > + goto out_unmap;
    > + }
    > +
    > + /*
    > + * If the page is mlock()d, we cannot swap it out.
    > + * If it's recently referenced (perhaps page_referenced
    > + * skipped over this mm) then we should reactivate it.
    > + */
    > if (vma->vm_flags & VM_LOCKED) {
    > ret = SWAP_MLOCK;
    > goto out_unmap;
    > @@ -790,7 +796,19 @@ static int try_to_unmap_one(struct page
    >
    > /* Nuke the page table entry. */
    > flush_cache_page(vma, address, page_to_pfn(page));
    > - pteval = ptep_clear_flush_notify(vma, address, pte);
    > + pteval = ptep_clear_flush(vma, address, pte);
    > +
    > + if (!migration) {
    > + /* re-check */
    > + if (PageSwapCache(page) &&
    > + page_count(page) != page_mapcount(page) + 2) {
    > + /* We lose race against get_user_pages_fast() */
    > + set_pte_at(mm, address, pte, pteval);
    > + ret = SWAP_FAIL;
    > + goto out_unmap;
    > + }
    > + }
    > + mmu_notifier_invalidate_page(vma->vm_mm, address);

    With regard to mmu notifier, this is the opposite of the right
    ordering. One mmu_notifier_invalidate_page must run _before_ the first
    check. The ptep_clear_flush_notify will then stay and there's no need
    of a further mmu_notifier_invalidate_page after the second check.


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