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X-IronPort-AV: E=McAfee;i="6600,9927,10974"; a="602421" X-IronPort-AV: E=Sophos;i="6.05,242,1701158400"; d="scan'208";a="602421" Received: from fmviesa007.fm.intel.com ([10.60.135.147]) by orvoesa110.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Feb 2024 18:58:00 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.05,242,1701158400"; d="scan'208";a="652545" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmviesa007-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Feb 2024 18:57:57 -0800 From: "Huang, Ying" To: Peng Zhang Cc: , , , , , , , , , Subject: Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault() In-Reply-To: <20240204093526.212636-1-zhangpeng362@huawei.com> (Peng Zhang's message of "Sun, 4 Feb 2024 17:35:26 +0800") References: <20240204093526.212636-1-zhangpeng362@huawei.com> Date: Mon, 05 Feb 2024 10:56:01 +0800 Message-ID: <87zfwf39ha.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 31CA914000F X-Stat-Signature: dmp8wdjdkxhr7hdqjttgkq39mxtfjjaz X-HE-Tag: 1707101881-866388 X-HE-Meta: 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 3GGG2B3z U2O39aWd66qJIP5qYCIZkRKYJj8AYzJ0U6eHY7Ycxboua8aMUc3fej+D0Fqb0iaroZIkojufw49V18b/O/6YQVIYH5vyOT9NyDc3hBklRZXZCihlCOZ/QYoRJF3BlZmLl+TnieZPEQXkz0EEXgJ6HKYECfgwY3zHIMaykU4vDkPrOvNkhClshL5j7Y49ns7LuzVuCX6e1x2rVfPXPEWquq3yHixe3bo6guTVp2ZxJJetVUSTlPj4YRPzfj0XmB6WHQLA5rkZII/USeQaqc5YRVv+DnOTEbfXDfONBQlssGvMDYqJ6W0qLpE3Dw8SeH7lM1alZVhb9UbujTO7nL8b8dcxWfg74/Nq+voIF 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: 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. -- 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);