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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 24884C61DF4 for ; Fri, 24 Nov 2023 07:26:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230305AbjKXH0i (ORCPT ); Fri, 24 Nov 2023 02:26:38 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229453AbjKXH0h (ORCPT ); Fri, 24 Nov 2023 02:26:37 -0500 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D66A8A3 for ; Thu, 23 Nov 2023 23:26:42 -0800 (PST) Received: from kwepemm000020.china.huawei.com (unknown [172.30.72.57]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4Sc5zw4ylCz1P8nL; Fri, 24 Nov 2023 15:23:08 +0800 (CST) Received: from [10.174.179.160] (10.174.179.160) 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 15:26:39 +0800 Message-ID: <513144c1-c0e0-fe63-e133-c2611a440a94@huawei.com> Date: Fri, 24 Nov 2023 15:26:38 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault() Content-Language: en-US To: "Huang, Ying" CC: , , , , , , , , , , References: <20231122140052.4092083-1-zhangpeng362@huawei.com> <87a5r4988r.fsf@yhuang6-desk2.ccr.corp.intel.com> From: "zhangpeng (AS)" In-Reply-To: <87a5r4988r.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.160] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) 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 On 2023/11/23 16:36, Huang, Ying wrote: > Peng Zhang writes: > >> 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. >> >> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ >> >> Signed-off-by: ZhangPeng >> Signed-off-by: Kefeng Wang > Suggested-by: "Huang, Ying" > > :-) > >> --- >> mm/filemap.c | 14 ++++++++++++++ >> 1 file changed, 14 insertions(+) >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index 71f00539ac00..bb5e6a2790dc 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >> mapping_locked = true; >> } >> } else { >> + pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, >> + vmf->address, &vmf->ptl); >> + if (ptep) { >> + /* >> + * 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 = VM_FAULT_NOPAGE; >> + pte_unmap_unlock(ptep, vmf->ptl); >> + if (unlikely(ret)) >> + return ret; >> + } >> + > Need to deal with ptep == NULL. Although that is high impossible. Maybe we don't need to deal with ptep == NULL, because it has been considered later in filemap_fault()? ptep == NULL means that the ptep has been replaced with a PMD entry. In this case, major fault is also required. > -- > Best Regards, > Huang, Ying > >> /* No page in the page cache at all */ >> count_vm_event(PGMAJFAULT); >> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); -- Best Regards, Peng