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    Subject[PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork
    Date
    The page can be included into collapse as long as it doesn't have extra
    pins (from GUP or otherwise).

    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    ---
    mm/khugepaged.c | 25 ++++++++++++++-----------
    1 file changed, 14 insertions(+), 11 deletions(-)

    diff --git a/mm/khugepaged.c b/mm/khugepaged.c
    index 57ff287caf6b..1e7e6543ebca 100644
    --- a/mm/khugepaged.c
    +++ b/mm/khugepaged.c
    @@ -581,11 +581,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
    }

    /*
    - * cannot use mapcount: can't collapse if there's a gup pin.
    - * The page must only be referenced by the scanned process
    - * and page swap cache.
    + * Check if the page has any GUP (or other external) pins.
    + *
    + * The page table that maps the page has been already unlinked
    + * from the page table tree and this process cannot get
    + * additinal pin on the page.
    + *
    + * New pins can come later if the page is shared across fork,
    + * but not for the this process. It is fine. The other process
    + * cannot write to the page, only trigger CoW.
    */
    - if (page_count(page) != 1 + PageSwapCache(page)) {
    + if (total_mapcount(page) + PageSwapCache(page) !=
    + page_count(page)) {
    unlock_page(page);
    result = SCAN_PAGE_COUNT;
    goto out;
    @@ -672,7 +679,6 @@ static void __collapse_huge_page_copy(pte_t *pte, struct page *page,
    } else {
    src_page = pte_page(pteval);
    copy_user_highpage(page, src_page, address, vma);
    - VM_BUG_ON_PAGE(page_mapcount(src_page) != 1, src_page);
    release_pte_page(src_page);
    /*
    * ptl mostly unnecessary, but preempt has to
    @@ -1206,12 +1212,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
    goto out_unmap;
    }

    - /*
    - * cannot use mapcount: can't collapse if there's a gup pin.
    - * The page must only be referenced by the scanned process
    - * and page swap cache.
    - */
    - if (page_count(page) != 1 + PageSwapCache(page)) {
    + /* Check if the page has any GUP (or other external) pins */
    + if (total_mapcount(page) + PageSwapCache(page) !=
    + page_count(page)) {
    result = SCAN_PAGE_COUNT;
    goto out_unmap;
    }
    --
    2.26.0
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