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Tue, 27 Jul 2021 23:41:54 +0000 Received: from BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a]) by BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a%6]) with mapi id 15.20.4352.031; Tue, 27 Jul 2021 23:41:54 +0000 Subject: Re: [PATCH 0/3] hugetlb: fix potential ref counting races To: linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Muchun Song , Michal Hocko , Oscar Salvador , David Hildenbrand , Matthew Wilcox , Naoya Horiguchi , Mina Almasry , Andrew Morton References: <20210710002441.167759-1-mike.kravetz@oracle.com> From: Mike Kravetz Message-ID: <22abb0f0-ce85-3567-3bfd-74dd1450c14f@oracle.com> Date: Tue, 27 Jul 2021 16:41:51 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210710002441.167759-1-mike.kravetz@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MWHPR2001CA0004.namprd20.prod.outlook.com (2603:10b6:301:15::14) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.123] (50.38.35.18) by MWHPR2001CA0004.namprd20.prod.outlook.com (2603:10b6:301:15::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4373.18 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=mAPLZ1K0; dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=pmW03iAN; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=rlDx52ZY; spf=none (imf19.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 205.220.177.32) smtp.mailfrom=mike.kravetz@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Stat-Signature: 5i8m5f1iggjpfjj7zftpnyck7sjzgeun X-HE-Tag: 1627429324-911675 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: Any additional comments on these patches/this approach? The first patch addressing this issue actually went into the 5.14 merge window as commit 7118fc2906e2 ("hugetlb: address ref count racing in prep_compound_gigantic_page"). All this code is very tricky and subtle. It addresses potential issues discovered by code analysis. I do not believe the races have ever been experienced in practice. If anyone has suggestions for a simpler or alternative approach, I would love to hear them. -- Mike Kravetz On 7/9/21 5:24 PM, Mike Kravetz wrote: > When Muchun Song brought up a potential issue with hugetlb ref counting[1], > I started looking closer at the code. hugetlbfs is the only code with it's > own specialized compound page destructor and taking special action when ref > counts drop to zero. Potential races happen in this unique handling of ref > counts. The following patches address these races when creating and > destroying hugetlb pages. > > These potential races have likely existed since the creation of > hugetlbfs. They certainly have been around for more than 10 years. > However, I am unaware of anyone actually hitting these races. It is > VERY unlikely than anyone will actually hit these races, but they do > exist. > > I could not think of an easy (or difficult) way to force these races. > Therefore, testing consisted of adding code to randomly increase ref > counts in strategic places. In this way, I was able to exercise all the > race handling code paths. > > [1] https://lore.kernel.org/linux-mm/CAMZfGtVMn3daKrJwZMaVOGOaJU+B4dS--x_oPmGQMD=c=QNGEg@mail.gmail.com/ > > Mike Kravetz (3): > hugetlb: simplify prep_compound_gigantic_page ref count racing code > hugetlb: drop ref count earlier after page allocation > hugetlb: before freeing hugetlb page set dtor to appropriate value > > mm/hugetlb.c | 137 ++++++++++++++++++++++++++++++++++++++------------- > 1 file changed, 104 insertions(+), 33 deletions(-) >