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Shutemov" X-Google-Original-From: "Kirill A. Shutemov" Received: by box.localdomain (Postfix, from userid 1000) id 6FEFD100D29; Fri, 27 Mar 2020 20:06:07 +0300 (+03) To: akpm@linux-foundation.org, Andrea Arcangeli Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: [PATCH 4/7] khugepaged: Allow to callapse a page shared across fork Date: Fri, 27 Mar 2020 20:05:58 +0300 Message-Id: <20200327170601.18563-5-kirill.shutemov@linux.intel.com> X-Mailer: git-send-email 2.26.0 In-Reply-To: <20200327170601.18563-1-kirill.shutemov@linux.intel.com> References: <20200327170601.18563-1-kirill.shutemov@linux.intel.com> MIME-Version: 1.0 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 --- 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)) { /* * 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); 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; }