linux-kernel - Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across fork

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Date:   Fri, 27 Mar 2020 14:19:50 -0400
From:   "Zi Yan" <zi.yan@...t.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     akpm@...ux-foundation.org,
        "Andrea Arcangeli" <aarcange@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across
 fork

On 27 Mar 2020, at 13:05, Kirill A. Shutemov wrote:

> 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@...ux.intel.com>
> ---
>  mm/khugepaged.c | 28 ++++++++++++++++------------
>  1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 39e0994abeb8..b47edfe57f7b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -581,18 +581,26 @@ 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)) {

Do you think having a function for this check would be better? Since the check is used three times.

>  			/*
>  			 * Drain pagevec and retry just in case we can get rid
>  			 * of the extra pin, like in swapin case.
>  			 */
>  			lru_add_drain();
>  		}
> -		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;
> @@ -680,7 +688,6 @@ 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);

Maybe replace it with this?

VM_BUG_ON_PAGE(page_mapcount(src_page) + PageSwapCache(src_page) != page_count(src_page), src_page);


>  			release_pte_page(src_page);
>  			/*
>  			 * ptl mostly unnecessary, but preempt has to
> @@ -1209,12 +1216,9 @@ 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

Thanks.

—
Best Regards,
Yan Zi

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