From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8965FC2BA19 for ; Mon, 13 Apr 2020 10:05:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B2DD20737 for ; Mon, 13 Apr 2020 10:05:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728564AbgDMKFo (ORCPT ); Mon, 13 Apr 2020 06:05:44 -0400 Received: from mga05.intel.com ([192.55.52.43]:63626 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728410AbgDMKFR (ORCPT ); Mon, 13 Apr 2020 06:05:17 -0400 IronPort-SDR: AShcXmRT+21nbrpiCG2DaAMYtuQQjLbTa3+Jeq2vAJVeaazlchxGA3k+9d9eYgmLy3V/XRae0c S8HaNru5LSSA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 13 Apr 2020 03:04:57 -0700 IronPort-SDR: cLZHmO+l+QKfmjoFjz3dZ1MGe5wuaWm/RPlyh8Ulv1B9c46mfYFCPsX0/iFhX7tTTOwgvuXTQ9 25V3V9bdAysQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,378,1580803200"; d="scan'208";a="426640509" Received: from black.fi.intel.com ([10.237.72.28]) by orsmga005.jf.intel.com with ESMTP; 13 Apr 2020 03:04:54 -0700 Received: by black.fi.intel.com (Postfix, from userid 1000) id A788B1F5; Mon, 13 Apr 2020 13:04:50 +0300 (EEST) From: "Kirill A. Shutemov" To: akpm@linux-foundation.org, Andrea Arcangeli Cc: Zi Yan , Yang Shi , Ralph Campbell , John Hubbard , William Kucharski , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: [PATCH 5/6] khugepaged: Allow to collapse a page shared across fork Date: Mon, 13 Apr 2020 13:04:46 +0300 Message-Id: <20200413100447.20073-6-kirill.shutemov@linux.intel.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20200413100447.20073-1-kirill.shutemov@linux.intel.com> References: <20200413100447.20073-1-kirill.shutemov@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- mm/khugepaged.c | 41 ++++++++++++++++++++++++++++++----------- 1 file changed, 30 insertions(+), 11 deletions(-) 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; } -- 2.26.0