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SubjectRe: [PATCH 4/7] khugepaged: Allow to callapse a page shared across fork
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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@linux.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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