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Subject: Re: [PATCHv3, RESEND 5/8] khugepaged: Allow to collapse a page shared
 across fork
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>
CC:     Zi Yan <ziy@nvidia.com>, Yang Shi <yang.shi@linux.alibaba.com>,
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References: <20200413125220.663-1-kirill.shutemov@linux.intel.com>
 <20200413125220.663-6-kirill.shutemov@linux.intel.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Tue, 14 Apr 2020 19:43:33 -0700
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On 2020-04-13 05:52, 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.
> 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 invariant it checks is no longer valid: the source can be mapped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(-)


Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

> 
> 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;
>   		}
>