On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> 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@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Acked-by: Jerome Marchand <jmarchan@xxxxxxxxxx>
> ---
> 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 0428e4c84e1d..17cdd6b9456b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -524,7 +524,8 @@ static inline int page_swapcount(struct page *page)
> 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 534f353e12bf..fd8af5b9917f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1103,7 +1103,17 @@ 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 6dd365d1c488..3cd5f188b996 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -887,6 +887,9 @@ 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);
>
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