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SubjectRe: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page shared across fork
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On 4/13/20 5:52 AM, Kirill A. Shutemov wrote:
> The page can be included into collapse as long as it doesn't have extra
> pins (from GUP or otherwise).
>
> Logic to check the refcound is moved to a separate function.

s/refcound/refcount

> Note that the function is ready to deal with compound pages. It's
> preparation for the following patch.
>
> VM_BUG_ON_PAGE() was removed from __collapse_huge_page_copy() as the
> invariant it checks is no longer valid: the source can be mapped
> multiple times now.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/khugepaged.c | 41 ++++++++++++++++++++++++++++++-----------
> 1 file changed, 30 insertions(+), 11 deletions(-)

Just a minor typo problem.

Acked-by: Yang Shi <yang.shi@linux.alibaba.com>

>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e3e41c2768d8..f9864644c3b7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -529,6 +529,24 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte)
> }
> }
>
> +static bool is_refcount_suitable(struct page *page)
> +{
> + int expected_refcount, refcount;
> +
> + refcount = page_count(page);
> + expected_refcount = total_mapcount(page);
> + if (PageSwapCache(page))
> + expected_refcount += compound_nr(page);
> +
> + if (IS_ENABLED(CONFIG_DEBUG_VM) && expected_refcount > refcount) {
> + pr_err("expected_refcount: %d, refcount: %d\n",
> + expected_refcount, refcount);
> + dump_page(page, "Unexpected refcount");
> + }
> +
> + return page_count(page) == expected_refcount;
> +}
> +
> static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> unsigned long address,
> pte_t *pte)
> @@ -581,11 +599,17 @@ 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
> + * an 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 (!is_refcount_suitable(page)) {
> unlock_page(page);
> result = SCAN_PAGE_COUNT;
> goto out;
> @@ -672,7 +696,6 @@ 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
> @@ -1201,12 +1224,8 @@ 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 (!is_refcount_suitable(page)) {
> result = SCAN_PAGE_COUNT;
> goto out_unmap;
> }

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