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Mon, 5 Feb 2024 15:24:59 +0800 Message-ID: <25de8872-ad79-e5e6-054c-9ac5e7191416@huawei.com> Date: Mon, 5 Feb 2024 15:24:59 +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] filemap: avoid unnecessary major faults in filemap_fault() Content-Language: en-US To: "Huang, Ying" CC: , , , , , , , , , References: <20240204093526.212636-1-zhangpeng362@huawei.com> <87zfwf39ha.fsf@yhuang6-desk2.ccr.corp.intel.com> <85e03dd9-8bd7-d516-ebe4-84dd449a9fb2@huawei.com> <87mssf2yiv.fsf@yhuang6-desk2.ccr.corp.intel.com> From: "zhangpeng (AS)" In-Reply-To: <87mssf2yiv.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: dggems706-chm.china.huawei.com (10.3.19.183) To kwepemm600020.china.huawei.com (7.193.23.147) X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: E6799C0007 X-Stat-Signature: py7se5xwe5qfyhy7jsu39gx5845sdeo1 X-HE-Tag: 1707117904-707096 X-HE-Meta: U2FsdGVkX1+XCtBTPCi+QZZQK3udbSfhRmqxpc0Gxf+2Iw/v2iEo5WB7AW5LeXQJ8B3Cwbb20hCG8xSET0kSdl+wnstFpq1oSTvMgo+2uTME+57GrlFm0dz9tb+FyCGRJlKZURMqLrKUuP6jk1cRQgnysEDF8L1Mq8berLG0cpLCQG8P0NSbocWcKVA2Msjml5DLe0LA346MrUEOPBSImd6/+ucln097TnGNr1U/ioxHS7I9R7dNeHkjsj4OHzZ7CslHieNHaECEfpogrnp3Dg0FS3phr14qij3ZBI+1j3qeskTsVhsykuGAX4emexOKmwG85bpSx5wMsIP4K+vIsmbANeHkYWJPsgjsmiJ4V3Llt84iFZT8oFWxNCNJwSttDYoYyj0Q2t5XgGPreIGXPCH9511uRQ8zP5wKvNPSMZEjKwGjG15qvKcrvO0f6XtVXGyzbzXPeK5oF1n8xb5lyH8fudKHsqefieTwYtl0ayoPu8e0ojxJvA0K30CWuHoXlovZVwcTAISFhXzY2x4w7bPzZ7OkAFZcSfminx/ySRhvsRWotBlt3wL6IeOGxgHlDOiDD1CCkM6Kjr6dET82K8JECHHzSEzA+eSXnsQdNcRKXDMuXu6ijvPw+NuhezsJtQ5C3H6Y1Fs6ppMchiul2gVTFzEXR9AINiGZtfBq/zp+4Ym3LGyipDmH/0iFmN0qGBSmqqT0YOH8PBbY2PHVZDUaF6obX8GWmFptTTjrRJI9ZNniCxMyqGyQ9JWrXc4YAshZx5A5o11ZkeaJZ5+lDqlvzWnZHJEkS0yXS8cHfQJGWG5QchztFqBs7LtFFs9mIYezyjonCiHS4Q81eVXyYwQDoMAHu5ukdyECfpYgj4mThACDgti7dSmIZu4jbHvNzTcO7IOHl3bcVfvf0NMLPla0qbkasA2AAqDFvV7B2l8qI0tFiqyeuhEjKTcAE1VXA29otpKUf0+FD6BMa3k OMlPjBjQ 6GJzDse+VVebMpkuWJWrsygRMgvzPwaARzvYGtCWXnBsY3s1RjPhKJeom7J4R9DD1PsPBL+4U8wyDUU3761hatBXRPaRMJ5NfE/u70lYiQ/maJkmI4uCopX1YIPHI71bJqdT8a5vp53U5Raq9Z0SXPYue1DPBJlGQpl8kFaGGiZLotISpwJNN3SiAUcX8YsmlomWvn0RBMIg0zVYFh0lBLpUXp3VDX5xxXql/udPSWcTnsdz6j320cY31UQlIH/67vgfpr4Lbegt+gHoL5wf/zy5CKPFEM5RSUsfWbPMStn84xXaT+gcj9kQhRoCLFlwr2ikIKSSgwqWZKDE= 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 2024/2/5 14:52, Huang, Ying wrote: > "zhangpeng (AS)" writes: >> On 2024/2/5 10:56, 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 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: >>>> processes processes_idle threads threads_idle >>>> ext4 file write: -1.14% -0.08% -1.87% 0.13% >>>> ext4 file read: 0.03% -0.65% -0.51% -0.08% >>>> ramdisk file write: -1.21% -0.21% -1.12% 0.11% >>>> ramdisk 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 >>>> --- >>>> 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 | 18 ++++++++++++++++++ >>>> 1 file changed, 18 insertions(+) >>>> >>>> diff --git a/mm/filemap.c b/mm/filemap.c >>>> index 142864338ca4..b29cdeb6a03b 100644 >>>> --- a/mm/filemap.c >>>> +++ b/mm/filemap.c >>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd, >>>> + vmf->address, &vmf->ptl); >>>> + if (unlikely(!ptep)) >>>> + return VM_FAULT_NOPAGE; >>>> + /* >>>> + * Recheck pte as the pte can be cleared temporarily >>>> + * during a read/modify/write update. >>>> + */ >>> I think that we should add some comments here about the racy checking. >> I'll add comments in a v2 as follows: >> /* >> * Recheck PTE as the PTE can be cleared temporarily >> * during a read/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. >> */ > Sorry, my previous words aren't clear enough. I mean some comments as > follows, > > We don't hold PTL here, so the check is still racy. But acquiring PTL > hurts performance and the race window seems small enough. Got it. I'll add comments in a v2 as follows: /* * Recheck PTE as the PTE can be cleared temporarily * during a read/modify/write update of the PTE. * We don't hold PTL here as acquiring PTL hurts * performance. So the check is still racy, but * the race window seems small enough. */ > > -- > Best Regards, > Huang, Ying > >>>> + 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, Peng