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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY,URIBL_BLOCKED,USER_AGENT_SANE_1 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 37D2CC2BA1A for ; Mon, 6 Apr 2020 20:51:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1C107206C3 for ; Mon, 6 Apr 2020 20:51:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726396AbgDFUvD (ORCPT ); Mon, 6 Apr 2020 16:51:03 -0400 Received: from out30-57.freemail.mail.aliyun.com ([115.124.30.57]:41673 "EHLO out30-57.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726225AbgDFUvC (ORCPT ); Mon, 6 Apr 2020 16:51:02 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R801e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01419;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0TuqY52Z_1586206257; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TuqY52Z_1586206257) by smtp.aliyun-inc.com(127.0.0.1); Tue, 07 Apr 2020 04:51:00 +0800 Subject: Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork To: "Kirill A. Shutemov" , akpm@linux-foundation.org, Andrea Arcangeli Cc: Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" References: <20200403112928.19742-1-kirill.shutemov@linux.intel.com> <20200403112928.19742-6-kirill.shutemov@linux.intel.com> From: Yang Shi Message-ID: Date: Mon, 6 Apr 2020 13:50:56 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20200403112928.19742-6-kirill.shutemov@linux.intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/3/20 4:29 AM, 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 > --- > mm/khugepaged.c | 25 ++++++++++++++----------- > 1 file changed, 14 insertions(+), 11 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 57ff287caf6b..1e7e6543ebca 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -581,11 +581,18 @@ 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)) { This check looks fine for base page, but what if the page is PTE-mapped THP? The following patch made this possible. If it is PTE-mapped THP and the page is in swap cache, the refcount would be 512 + the number of PTE-mapped pages. Shall we do the below change in the following patch? extra_pins = PageSwapCache(page) ? nr_ccompound(page) - 1 : 0; if (total_mapcount(page) + PageSwapCache(page) != page_count(page) - extra_pins) { ... } > unlock_page(page); > result = SCAN_PAGE_COUNT; > goto out; > @@ -672,7 +679,6 @@ 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 > @@ -1206,12 +1212,9 @@ 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; > }