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Mon, 5 Feb 2024 15:36:02 +0800 Message-ID: Date: Mon, 5 Feb 2024 15:36:01 +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: David Hildenbrand , "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> <25de8872-ad79-e5e6-054c-9ac5e7191416@huawei.com> <1a967b8d-7218-4d3f-9dd2-ae1c66f626c7@redhat.com> From: "zhangpeng (AS)" In-Reply-To: <1a967b8d-7218-4d3f-9dd2-ae1c66f626c7@redhat.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit 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-Stat-Signature: s87o3g53jd8h9794k6onppqn3i4rjj3x X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 307F140007 X-HE-Tag: 1707118567-538168 X-HE-Meta: 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 OtKupBOC utPPs4LBEIf30tudGV/ZjsMskuS8uvEEWvho8B7VbEl7TV0rGAjEXhTUN1CZGV68HwRF764NQSJsnyHBqOLJqGKX2SrGVuDL7ksEiLmBIfU7Cf/ZtHDVYyJtB8ZWjTNGbQT/MX2ueLcDqOj8Cy8F2ML4wHL6yUq/6opWsErJNZvCjeor4KqL+f01O19WwwJJDiqifVOzMezLZsDUIy9tNVBQDYEFdryPUQ575FtD0bvFfB8dnCk7P3pb6vBNoTQgKwcNtDfy07s+t4Cy/1mf2XoPtuKz9MMFxsSHeZ9p/6qdUy70SECEuz+iwVZra1BIUmYJ5EDnAV4A8WI8= 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 15:31, David Hildenbrand wrote: > On 05.02.24 08:24, zhangpeng (AS) wrote: >> 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. >>    */ > > It'd be worth spelling out what happens when we lose the race. > I'll add what happens when we lose the race 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. * 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, Peng