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    Subject[PATCHv12 04/37] mm, thp: adjust conditions when we can reuse the page on WP fault
    Date
    With new refcounting we will be able map the same compound page with
    PTEs and PMDs. It requires adjustment to conditions when we can reuse
    the page on write-protection fault.

    For PTE fault we can't reuse the page if it's part of huge page.

    For PMD we can only reuse the page if nobody else maps the huge page or
    it's part. We can do it by checking page_mapcount() on each sub-page,
    but it's expensive.

    The cheaper way is to check page_count() to be equal 1: every mapcount
    takes page reference, so this way we can guarantee, that the PMD is the
    only mapping.

    This approach can give false negative if somebody pinned the page, but
    that doesn't affect correctness.

    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Tested-by: Sasha Levin <sasha.levin@oracle.com>
    Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
    Acked-by: Jerome Marchand <jmarchan@redhat.com>
    Acked-by: Vlastimil Babka <vbabka@suse.cz>
    ---
    include/linux/swap.h | 3 ++-
    mm/huge_memory.c | 12 +++++++++++-
    mm/swapfile.c | 3 +++
    3 files changed, 16 insertions(+), 2 deletions(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index 9c7c4b418498..1184fdbd30ba 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -540,7 +540,8 @@ static inline int swp_swapcount(swp_entry_t entry)
    return 0;
    }

    -#define reuse_swap_page(page) (page_mapcount(page) == 1)
    +#define reuse_swap_page(page) \
    + (!PageTransCompound(page) && page_mapcount(page) == 1)

    static inline int try_to_free_swap(struct page *page)
    {
    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index 7c41c0606e98..76a845b12d8d 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -1230,7 +1230,17 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,

    page = pmd_page(orig_pmd);
    VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
    - if (page_mapcount(page) == 1) {
    + /*
    + * We can only reuse the page if nobody else maps the huge page or it's
    + * part. We can do it by checking page_mapcount() on each sub-page, but
    + * it's expensive.
    + * The cheaper way is to check page_count() to be equal 1: every
    + * mapcount takes page reference reference, so this way we can
    + * guarantee, that the PMD is the only mapping.
    + * This can give false negative if somebody pinned the page, but that's
    + * fine.
    + */
    + if (page_mapcount(page) == 1 && page_count(page) == 1) {
    pmd_t entry;
    entry = pmd_mkyoung(orig_pmd);
    entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
    diff --git a/mm/swapfile.c b/mm/swapfile.c
    index f131bc1838d3..c6aec93c8c0b 100644
    --- a/mm/swapfile.c
    +++ b/mm/swapfile.c
    @@ -929,6 +929,9 @@ int reuse_swap_page(struct page *page)
    VM_BUG_ON_PAGE(!PageLocked(page), page);
    if (unlikely(PageKsm(page)))
    return 0;
    + /* The page is part of THP and cannot be reused */
    + if (PageTransCompound(page))
    + return 0;
    count = page_mapcount(page);
    if (count <= 1 && PageSwapCache(page)) {
    count += page_swapcount(page);
    --
    2.5.3


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