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Mon, 5 Feb 2024 14:43:08 +0800 Message-ID: <85e03dd9-8bd7-d516-ebe4-84dd449a9fb2@huawei.com> Date: Mon, 5 Feb 2024 14:43:07 +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> From: "zhangpeng (AS)" In-Reply-To: <87zfwf39ha.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: rspam12 X-Rspamd-Queue-Id: 8EE1018000C X-Stat-Signature: pcfsfd8k4ek3pjtpae5b114bfyeur89w X-HE-Tag: 1707115392-625427 X-HE-Meta: U2FsdGVkX19PReIxPj7Eo4CPBAnmOAKYDZ2baneidZpjFGpek+uD1hljN6LW+6L5CvMGTFHfpTZ0Fd3/y7DvbnoAaJEsJ0u/NO4YqOgQR2xk3lp9qTcyG173RC2mctyOd2u0Wo9af28nLo9DEdvuufxKaUhdgnzt6eKOggRJXUs+lXWCbnvOGoOqGmEat8vYI5DaOnsrsu2vsvwg8+YltjAJCENlddYJgQCdpwkCG0spFjNW3KztdZCpQlG2rQPkjFUhqUA0mPsjMjCZz7IFdVMW4om6tO1BezQiAsHoQM3dt4c8G64+xLUD37C0S96zNrPhKIdJv2YXEbrMqR+OZNuPpQ6Epq+0Tpe9JQB79TAq1m3xxqUr3k3FkL4Ge/FGgQ64KxnM0rJpev02TeWACUpFwTqHgxNioaAr6z83cJpaXGt7m67B4tPbZGpEaiQbame51/Q97cQ4aqoCDvUQLBc+gOi9NOUlnwUSO7lFjo2DGPL5z/XEo+5ScxDvfzuDXEwcgGYnfj18kqZ+CED9lk7b5vznI00djSBCBVH1sGdvGAcK4SA/LvtvHZ+cgufCXgeQeH8qUcC3yVydu/8S4WXY+ZfqVPixssiLKR7Go1f4LDOB2mDjmGfkg88oa7X5ySowItu3Ad6gVMO6ckTSh6ZpNGEeZVO7L8cG/AoY7MCgT2k+/0IVYS7IvHqiiS02zU6+2t8IIHEydVQy+03sD8G1S3U9EX5v3cRHLD21WNZ6fnf4J/DE+35bVOdvRzO5eGIQzc995pVhD2Z6aNn2WhH1aPJJ2IN0X9ljQPNXw1YOLKLPauwvlsqfgIaxr2WIjlg7ZiQwCIuRtsqr79nSe8utcz76kqtzJ4c7coqtOU+4QOOvNis8Z5uveZ4ryd/I5+XvruHqbk7Wu16b29GgmkAbulSoMYYz7TJHLzlKJJ+ROvfLoL837tq6gdo+VuNVCjImgoEteAqmLjxdwCZ 1Nb9cnHt sqjUVOjsrPL3y32NUUbU1MkpLtBWmbXzuavlv1/h+vTsefNNGmwweaTjjQSt9bnk2BfkoZ0izDIf/UkIQlYhf9iGh50GofxXUafpaYvsrgUDot0R5dOw2HhfvV9oHJ2CHAcPnh92bZNmkH3i3S18ltimMLLU65VcPR50A/l5rvmrfuiMCrJclLH7LjkW9bjM1wld7Yebd2rNMa/TmsKQjlAj6cTpzuXaS7KlYOsw/ySrNq7KXY3mUccisXpeqag1qoiBiIT3AlA5vbVTpZ9vTNMNftyd2l/K0jXuI1rcIN3y0SZ0yFgEyppimRvBmfN4FPPeh9BF+Gx49IiY= 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 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. */ > > -- > 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