From nobody Wed Dec 17 15:36:26 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1B4ABC61D85 for ; Fri, 24 Nov 2023 02:31:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230466AbjKXCbN (ORCPT ); Thu, 23 Nov 2023 21:31:13 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46142 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229478AbjKXCbL (ORCPT ); Thu, 23 Nov 2023 21:31:11 -0500 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D1D5A1BD for ; Thu, 23 Nov 2023 18:31:17 -0800 (PST) Received: from kwepemm000020.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4SbzR40YJ6zsRPs; Fri, 24 Nov 2023 10:27:44 +0800 (CST) Received: from localhost.localdomain (10.175.112.125) by kwepemm000020.china.huawei.com (7.193.23.93) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Fri, 24 Nov 2023 10:31:14 +0800 From: Peng Zhang To: , CC: , , , , , , , , , , ZhangPeng Subject: [PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault() Date: Fri, 24 Nov 2023 10:31:07 +0800 Message-ID: <20231124023107.571059-1-zhangpeng362@huawei.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Originating-IP: [10.175.112.125] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To kwepemm000020.china.huawei.com (7.193.23.93) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: ZhangPeng The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) in application, which leading to an unexpected performance issue[1]. This caused by temporarily cleared pte during a read/modify/write update of the pte, eg, do_numa_page()/change_pte_range(). For the data segment of the user-mode program, the global variable area is a private mapping. After the pagecache is loaded, the private anonymous page is generated after the COW is triggered. Mlockall can lock COW pages (anonymous pages), but the original file pages cannot be locked and may be reclaimed. If the global variable (private anon page) is accessed when vmf->pte is zeroed in numa fault, a file page fault will be triggered. At this time, the original private file page may have been reclaimed. If the page cache is not available at this time, a major fault will be triggered and the file will be read, causing additional overhead. Fix this by rechecking the pte by holding ptl in filemap_fault() before triggering a major fault. We tested the performance of file private mapping page fault (page_fault2.c of will-it-scale [2]) and file shared mapping page fault (page_fault3.c of will-it-scale). The difference in performance (in operations per second) before and after patch applied is about 0.7% on a x86 physical machine. [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@h= uawei.com/ [2] https://github.com/antonblanchard/will-it-scale/tree/master Suggested-by: "Huang, Ying" Signed-off-by: ZhangPeng Signed-off-by: Kefeng Wang Reviewed-by: Yin Fengwei --- RFC->v1: - Update commit message and add RB from Yin Fengwei - Add error handling when ptep =3D=3D NULL per Huang, Ying and Matthew Wilcox mm/filemap.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/mm/filemap.c b/mm/filemap.c index 71f00539ac00..f3dcabdbc810 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) mapping_locked =3D true; } } else { + pte_t *ptep =3D pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, + vmf->address, &vmf->ptl); + if (unlikely(!ptep)) + return VM_FAULT_NOPAGE; + /* + * Recheck pte with ptl locked as the pte can be cleared + * temporarily during a read/modify/write update. + */ + if (unlikely(!pte_none(ptep_get(ptep)))) + ret =3D VM_FAULT_NOPAGE; + pte_unmap_unlock(ptep, vmf->ptl); + if (unlikely(ret)) + return ret; + /* No page in the page cache at all */ count_vm_event(PGMAJFAULT); count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); --=20 2.25.1