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Tue, 11 May 2021 16:38:14 +0000 Subject: Re: resv_huge_page underflow with userfaultfd test To: Axel Rasmussen , Mina Almasry Cc: Linux-MM , Andrea Arcangeli , Peter Xu References: From: Mike Kravetz Message-ID: <5d4eb1c7-0e46-7b71-b692-72cee179e0aa@oracle.com> Date: Tue, 11 May 2021 09:38:12 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [50.38.35.18] X-ClientProxiedBy: MW4PR03CA0087.namprd03.prod.outlook.com (2603:10b6:303:b6::32) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MW4PR03CA0087.namprd03.prod.outlook.com (2603:10b6:303:b6::32) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4108.25 via Frontend Transport; Tue, 11 May 2021 16:38:13 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 3f7da8d3-4c4e-498d-aa99-08d9149b2c02 X-MS-TrafficTypeDiagnostic: BYAPR10MB2902: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:6790; 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dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=Qmcq86YF; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b="e/wuQKz/"; dmarc=pass (policy=none) header.from=oracle.com; spf=none (imf19.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 156.151.31.85) smtp.mailfrom=mike.kravetz@oracle.com X-Stat-Signature: zjqwq3uim6gn9xpgjrkabmt9kdfroeh1 X-Rspamd-Queue-Id: 800E090009EA X-Rspamd-Server: rspam02 Received-SPF: none (oracle.com>: No applicable sender policy available) receiver=imf19; identity=mailfrom; envelope-from=""; helo=userp2120.oracle.com; client-ip=156.151.31.85 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620751066-595567 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000061, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 5/10/21 11:45 PM, Axel Rasmussen wrote: > Thanks for the investigation, Mike! > > Mina, since hugetlb_mcopy_atomic_pte is specific to userfaultfd, I'm > happy to take a deeper look at it this week as well. > > For context, we have seen the WARN_ON Mina described trigger in > production before, but were never able to reproduce it. The > userfaultfd self test turns out to reproduce it reliably, so the > thinking up to this point was that it just happened to reproduce some > non-userfaultfd-specific issue. But from Mike's description, it seems > this bug is very specific to userfaultfd after all. :) Certainly, this case is userfaultfd specific. However, I too recall seeing 'transient' underflows in the past. Pretty sure this was not userfaultfd specific. Specifically, when working on commit 22146c3ce989 "hugetlbfs: dirty pages as they are added to pagecache" I recall seeing transient underflow. After fixing the issue in 22146c3ce989, I could not reproduce transient underflows and stopped looking for the cause. We added code to a production kernel in an attmempt to catch the issue: https://github.com/oracle/linux-uek/commit/bd697676290d91762ef2bf79832f653b44e6f83b#diff-fb6066ca63d9afdc2e3660c85e2dcc04cf31b37900ca9df2a1019ee8fa80dce0 I'll try running some non-userfaultfd specific tests to see if I can reproduce. -- Mike Kravetz