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Tue, 16 Nov 2021 03:31:39 +0000 Message-ID: <36bb52ee-6cc4-77f4-c99e-541741413f2d@oracle.com> Date: Mon, 15 Nov 2021 21:31:34 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 Subject: Re: Question regarding map count for compound pages Content-Language: en-US To: Harish Mara , linux-mm@kvack.org Cc: Aniket Kulkarni , Pawan Powar , Rajshekar Iyer , ask@linux.vnet.ibm.com References: From: John Donnelly In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YT1PR01CA0047.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:2e::16) To CO1PR10MB4722.namprd10.prod.outlook.com (2603:10b6:303:9e::12) MIME-Version: 1.0 Received: from [192.168.1.106] (47.220.27.44) by YT1PR01CA0047.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:2e::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4690.26 via Frontend Transport; Tue, 16 Nov 2021 03:31:37 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: bf1dc853-9df6-44a6-4702-08d9a8b19991 X-MS-TrafficTypeDiagnostic: MWHPR10MB1597: X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:10000; 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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=g+LkXVdr; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b="NbmzoLN/"; spf=none (imf25.hostedemail.com: domain of john.p.donnelly@oracle.com has no SPF policy when checking 205.220.165.32) smtp.mailfrom=john.p.donnelly@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: E2157B00019C X-HE-Tag: 1637033493-321305 X-Bogosity: Ham, tests=bogofilter, spamicity=0.004581, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 11/15/21 20:47, Harish Mara wrote: > Background: > On older kernel?s, we could have our device driver create char devices and > implement file_operations and vm_operations for open, release, mmap, fault > etc. The driver allocates memory (128KB, order 5) as compound pages. > The user application would then map/mmap these device files to perform > read/write operations. > > On recent kernels, this creates problems when the user space maps multiple > 128KB chunks that exceed 2MB. This would sometime result in bad page > getting mapped to the user space process. Almost always we see ?Bad page > map? errors during munmap because the map count is going below 0. > > It looks like the culprit is zap_pte_range(), which calls > page_remove_rmap() with the compound_flag = false. As a result, instead of > decrementing the compound_mapcount for the page the page->_mapcount is > decremented causing a lot of bad page errors. > > Questions: > Is this the right usage of compound pages? I.e can I allocate compound > pages in my kernel driver to be mapped to char device file by a user > process? > If yes, then why does it fail on latest kernel when the mmap-ed size > exceeds 2MB? > If no, why was it working on the older kernels? If it worked then, > shouldn?t it work now? > What is special about map size being greater than 2MB to trigger this? > Should compound pages be used for Anonymous purposes only? > > Hi. It would certainly help if you could isolate when this behavior started . Which version x.y.z version works ? And which one fail ?