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 8C8DFC5AD4C for ; Thu, 23 Nov 2023 07:57:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344896AbjKWH5p (ORCPT ); Thu, 23 Nov 2023 02:57:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44166 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234509AbjKWH5n (ORCPT ); Thu, 23 Nov 2023 02:57:43 -0500 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 34EFED43 for ; Wed, 22 Nov 2023 23:57:48 -0800 (PST) Received: from kwepemm000020.china.huawei.com (unknown [172.30.72.53]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4SbVkH5xnrz1P8YT; Thu, 23 Nov 2023 15:54:15 +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; Thu, 23 Nov 2023 15:57:44 +0800 Message-ID: <48235d73-3dc6-263d-7822-6d479b753d46@huawei.com> Date: Thu, 23 Nov 2023 15:57:44 +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: Yin Fengwei , , CC: , , , , , , , , References: <20231122140052.4092083-1-zhangpeng362@huawei.com> <801bd0c9-7d0c-4231-93e5-7532e8231756@intel.com> From: "zhangpeng (AS)" In-Reply-To: <801bd0c9-7d0c-4231-93e5-7532e8231756@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit 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 13:26, Yin Fengwei wrote: > On 11/23/23 12:12, zhangpeng (AS) wrote: >> On 2023/11/23 9:09, Yin Fengwei wrote: >> >>> Hi Peng, >>> >>> On 11/22/23 22:00, Peng Zhang wrote: >>>> 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 >>>> --- >>>>   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; >>>> +        } >>> I am curious. Did you try not to take PTL here and just check whether PTE is not NONE? >> Thank you for your reply. >> >> If we don't take PTL, the current use case won't trigger this issue either. > Is this verified by testing or just in theory? If we add a delay between ptep_modify_prot_start() and ptep_modify_prot_commit(), this issue will also trigger. Without delay, we haven't reproduced this problem so far. >> In most cases, if we don't take PTL, this issue won't be triggered. However, >> there is still a possibility of triggering this issue. The corner case is that >> task 2 triggers a page fault when task 1 is between ptep_modify_prot_start() >> and ptep_modify_prot_commit() in do_numa_page(). Furthermore,task 2 passes the >> check whether the PTE is not NONE before task 1 updates PTE in >> ptep_modify_prot_commit() without taking PTL. > There is very limited operations between ptep_modify_prot_start() and > ptep_modify_prot_commit(). While the code path from page fault to this check is > long. My understanding is it's very likely the PTE is not NONE when do PTE check > here without hold PTL (This is my theory. :)). Yes, there is a high probability that this issue won't occur without taking PTL. > In the other side, acquiring/releasing PTL may bring performance impaction. It may > not be big deal because the IO operations in this code path. But it's better to > collect some performance data IMHO. We tested the performance of file private mapping page fault (page_fault2.c of will-it-scale [1]) 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://github.com/antonblanchard/will-it-scale/tree/master > > Regards > Yin, Fengwei > >>> Regards >>> Yin, Fengwei >>> >>>> + >>>>           /* No page in the page cache at all */ >>>>           count_vm_event(PGMAJFAULT); >>>>           count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); -- Best Regards, Peng