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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 623F7C48BF6 for ; Mon, 26 Feb 2024 08:19:58 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8564E6B0174; Mon, 26 Feb 2024 03:19:57 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 807506B0175; Mon, 26 Feb 2024 03:19:57 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6CDFE6B0176; Mon, 26 Feb 2024 03:19:57 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 5E4F56B0174 for ; Mon, 26 Feb 2024 03:19:57 -0500 (EST) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 34E741A06CD for ; Mon, 26 Feb 2024 08:19:57 +0000 (UTC) X-FDA: 81833256834.07.4CD4420 Received: from szxga06-in.huawei.com (szxga06-in.huawei.com [45.249.212.32]) by imf11.hostedemail.com (Postfix) with ESMTP id AAF2540012 for ; Mon, 26 Feb 2024 08:19:53 +0000 (UTC) Authentication-Results: imf11.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf11.hostedemail.com: domain of zhangpeng362@huawei.com designates 45.249.212.32 as permitted sender) smtp.mailfrom=zhangpeng362@huawei.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1708935595; a=rsa-sha256; cv=none; b=cXHCwdYcJLPWfqhQI0kTPm7b8TJxlUZEd5gBHmvKQRvqV/cpYLdm8WwmJrLnHPSt2SJ6m9 HP97LOCc7wlyGz1Zl/SEyEBO3fyLT2LQrWBcS5QdzUtEoCSwZpsuR9HH0pgwmA/LVkt5EJ 2mmS+Lx+Tc9ITuvVKekvTm8kG8wd8sY= ARC-Authentication-Results: i=1; imf11.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf11.hostedemail.com: domain of zhangpeng362@huawei.com designates 45.249.212.32 as permitted sender) smtp.mailfrom=zhangpeng362@huawei.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1708935595; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bXk5pL3lcZhYmgQ3MIp9bSU1FiGsgFrQDAYtPiyUYBM=; b=8MokFB+PwCDbeMZJFn5+jpTrSVcvrkxsn0MX0GJHoIrIJyxi70ss1nV29/QBlqlveZABbJ js3AtnonUA4lvVprUPWSzNhXIFhCj70oVe0fAp0KcadQIzkiQR1XgXlp4mE3mANkWfOETf mnM5LpFKAQnbeK8eZd7348/0GhY4M6c= Received: from mail.maildlp.com (unknown [172.19.88.234]) by szxga06-in.huawei.com (SkyGuard) with ESMTP id 4TjtnC6vDvzqjhN; Mon, 26 Feb 2024 16:19:11 +0800 (CST) Received: from kwepemm600020.china.huawei.com (unknown [7.193.23.147]) by mail.maildlp.com (Postfix) with ESMTPS id 40CAD14011F; Mon, 26 Feb 2024 16:19:48 +0800 (CST) Received: from [10.174.179.160] (10.174.179.160) by kwepemm600020.china.huawei.com (7.193.23.147) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Mon, 26 Feb 2024 16:19:47 +0800 Message-ID: Date: Mon, 26 Feb 2024 16:19:46 +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: [PATCH v2] filemap: avoid unnecessary major faults in filemap_fault() Content-Language: en-US To: "Huang, Ying" CC: , , , , , , , , , , Nanyong Sun References: <20240206092627.1421712-1-zhangpeng362@huawei.com> <87jznhypxy.fsf@yhuang6-desk2.ccr.corp.intel.com> <87frxfhibt.fsf@yhuang6-desk2.ccr.corp.intel.com> <43182940-ddaa-7073-001a-e95d0999c5ba@huawei.com> <87il2bek6f.fsf@yhuang6-desk2.ccr.corp.intel.com> From: "zhangpeng (AS)" In-Reply-To: <87il2bek6f.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: dggems704-chm.china.huawei.com (10.3.19.181) To kwepemm600020.china.huawei.com (7.193.23.147) X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: AAF2540012 X-Stat-Signature: f6fbd7hc9pjxaiia631cwjf6ci6n1ibt X-Rspam-User: X-HE-Tag: 1708935593-12568 X-HE-Meta: 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 +hgFH+at A+laCV1DG+b10PbN859GHdaob3mho0Qzc9ECt2tuSjqqqb50NBIsRTmXJimC1SOePDli3LVbOLiOR6LMwhNuIoSdqFfox+pks0Faf8i9EdQuYfJEE23FKABEDYYS6sqDFo0H/pJLWvadZtmK3qIaf/5tmkG5857BX6d2tm4Rr57j/UqkYMvZq8rqE6eR/j1F0niqSJ41NMEIRDTmiKZTnmQg5arFJHARBhlND6JfaKPOzKvcny9H5v3avEFgP9QylU0iI92vxWAoa3/2fL6hrtpYcl6alx8Qc/k9gsXGkVRnjULAjhKBSDDhqqj+i1jgmkfHfgjQR4zbgGhg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2024/2/26 15:52, Huang, Ying wrote: > "zhangpeng (AS)" writes: > >> On 2024/2/26 14:04, Huang, Ying wrote: >> >>> "zhangpeng (AS)" writes: >>> >>>> On 2024/2/7 10:21, 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+clear/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 without acquiring PTL in filemap_fault() >>>>>> before triggering a major fault. >>>>>> >>>>>> Testing file anonymous page read and write page fault performance in ext4 >>>>>> and ramdisk using will-it-scale[2] on a x86 physical machine. The data >>>>>> is the average change compared with the mainline after the patch is >>>>>> applied. The test results are within the range of fluctuation, and there >>>>>> is no obvious difference. The test results are as follows: >>>>> You still claim that there's no difference in the test results. If so, >>>>> why do you implement the patch? IMHO, you need to prove your patch can >>>>> improve the performance in some cases. >>>> I'm sorry that maybe I didn't express myself clearly. >>>> >>>> The purpose of this patch is to fix the issue that major fault may still be triggered >>>> with mlockall(), thereby improving a little performance. This patch is more of a bugfix >>>> than a performance improvement patch. >>>> >>>> This issue affects our traffic analysis service. The inbound traffic is heavy. If a major >>>> fault occurs, the I/O schedule is triggered and the original I/O is suspended. Generally, >>>> the I/O schedule is 0.7 ms. If other applications are operating disks, the system needs >>>> to wait for more than 10 ms. However, the inbound traffic is heavy and the NIC buffer is >>>> small. As a result, packet loss occurs. The traffic analysis service can't tolerate packet >>>> loss. >>>> >>>> To prevent packet loss, we use the mlockall() function to prevent I/O. It is unreasonable >>>> that major faults will still be triggered after mlockall() is used. >>>> >>>> In our service test environment, the baseline is 7 major faults/12 hours. After applied the >>>> unlock patch, the probability of triggering the major fault is 1 major faults/12 hours. After >>>> applied the lock patch, no major fault will be triggered. So only the locked patch can actually >>>> solve our problem. >>> This is the data I asked for. >>> >>> But, you said that this is a feature bug fix instead of performance >>> improvement. So, I checked the mlock(2), and found the following words, >>> >>> " >>> mlockall() locks all pages mapped into the address space of the calling >>> process. This includes the pages of the code, data, and stack segment, >>> as well as shared libraries, user space kernel data, shared memory, and >>> memory-mapped files. All mapped pages are guaranteed to be resident in >>> RAM when the call returns successfully; the pages are guaranteed to >>> stay in RAM until later unlocked. >>> " >>> >>> In theory, the locked page are in RAM. So, IIUC, we don't violate the >>> ABI. But, in effect, we does do that. >> "mlockall() locks all pages mapped into the address space of the calling process." >> For a private mapping, mlockall() can lock COW pages (anonymous pages), but the >> original file pages can't be locked. Maybe, we violate the ABI here. > If so, please make it explicit and loudly. > >> This is also >> the cause of this issue. The patch fix the impact of this issue: prevent major >> faults, reduce IO, and fix the service packet loss issue. >> >> Preventing major faults, and thus reducing IO, could be an important reason to use >> mlockall(). Could we fix this with the locked patch? Or is there another way? > Unfortunately, locked patch cause performance regressions for more > common cases. Is it possible for us to change ptep_modify_prot_start() > to use some magic PTE value instead of 0? That may be possible. But, > that isn't enough, you need to change all ptep_get_and_clear() users. Thank you very much for your suggestion! I'll try it this way. > -- > Best Regards, > Huang, Ying > >>> But, from git history, we have cleared the PTE during modification from >>> 2.6.12-rc2 at least. I guess that because Linux isn't a hard real time >>> OS, users don't expect that too. >>> >>> -- >>> Best Regards, >>> Huang, Ying >>> >>>> The test data provided is intended to prove that the patch does not have a major impact >>>> on the performance of the page fault itself. >>>> >>>>>> processes processes_idle threads threads_idle >>>>>> ext4 private file write: -1.14% -0.08% -1.87% 0.13% >>>>>> ext4 shared file write: 0.14% -0.53% 2.88% -0.77% >>>>>> ext4 private file read: 0.03% -0.65% -0.51% -0.08% >>>>>> tmpfs private file write: -0.34% -0.11% 0.20% 0.15% >>>>>> tmpfs shared file write: 0.96% 0.10% 2.78% -0.34% >>>>>> ramdisk private file write: -1.21% -0.21% -1.12% 0.11% >>>>>> ramdisk private file read: 0.00% -0.68% -0.33% -0.02% >>>>>> >>>>>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ >>>>>> [2] https://github.com/antonblanchard/will-it-scale/ >>>>>> >>>>>> Suggested-by: "Huang, Ying" >>>>>> Suggested-by: Yin Fengwei >>>>>> Signed-off-by: ZhangPeng >>>>>> Signed-off-by: Kefeng Wang >>>>>> --- >>>>>> v1->v2: >>>>>> - Add more test results per Huang, Ying >>>>>> - Add more comments before check PTE per Huang, Ying, David Hildenbrand >>>>>> and Yin Fengwei >>>>>> - Change pte_offset_map_nolock to pte_offset_map as the ptl lock won't >>>>>> be used >>>>>> >>>>>> RFC->v1: >>>>>> - Add error handling when ptep == NULL per Huang, Ying and Matthew >>>>>> Wilcox >>>>>> - Check the PTE without acquiring PTL in filemap_fault(), suggested by >>>>>> Huang, Ying and Yin Fengwei >>>>>> - Add pmd_none() check before PTE map >>>>>> - Update commit message and add performance test information >>>>>> >>>>>> mm/filemap.c | 34 ++++++++++++++++++++++++++++++++++ >>>>>> 1 file changed, 34 insertions(+) >>>>>> >>>>>> diff --git a/mm/filemap.c b/mm/filemap.c >>>>>> index 142864338ca4..a2c1a98bc771 100644 >>>>>> --- a/mm/filemap.c >>>>>> +++ b/mm/filemap.c >>>>>> @@ -3238,6 +3238,40 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >>>>>> mapping_locked = true; >>>>>> } >>>>>> } else { >>>>>> + if (!pmd_none(*vmf->pmd)) { >>>>>> + pte_t *ptep; >>>>>> + >>>>>> + ptep = pte_offset_map(vmf->pmd, vmf->address); >>>>>> + if (unlikely(!ptep)) >>>>>> + return VM_FAULT_NOPAGE; >>>>>> + /* >>>>>> + * Recheck PTE as the PTE can be cleared temporarily >>>>>> + * during a read+clear/modify/write update of the PTE, >>>>>> + * eg, do_numa_page()/change_pte_range(). This will >>>>>> + * trigger a major fault, even if we use mlockall, >>>>>> + * which may affect performance. >>>>>> + * We don't hold PTL here as acquiring PTL hurts >>>>>> + * performance. So the check is still racy, but >>>>>> + * the race window seems small enough. >>>>>> + * >>>>>> + * If we lose the race during the check, the page_fault >>>>>> + * will be triggered. Butthe page table entry lock >>>>>> + * still make sure the correctness: >>>>>> + * - If the page cache is not reclaimed, the page_fault >>>>>> + * will work like the page fault was served already >>>>>> + * and bail out. >>>>>> + * - If the page cache is reclaimed, the major fault >>>>>> + * will be triggered, page cache is filled, >>>>>> + * page_fault also work like the page fault was >>>>>> + * served already and bail out. >>>>>> + */ >>>>> IMHO, this is too long. It can be shorten to like, >>>>> >>>>> If we lose the race, major fault may be triggered unnecessary. This >>>>> hurts performance but not functionality. >>>> OK, I'll fix it in the next version. >>>> >>>>>> + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) >>>>>> + ret = VM_FAULT_NOPAGE; >>>>>> + pte_unmap(ptep); >>>>>> + 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); >>>>> -- >>>>> Best Regards, >>>>> Huang, Ying -- Best Regards, Peng