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Berrange" , Alex Williamson , Zefan Li , Andrew Morton Subject: Re: [PATCH 14/19] mm: Introduce a cgroup for pinned memory Message-ID: References: Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MN2PR02CA0015.namprd02.prod.outlook.com (2603:10b6:208:fc::28) To LV2PR12MB5869.namprd12.prod.outlook.com (2603:10b6:408:176::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LV2PR12MB5869:EE_|CH2PR12MB4922:EE_ X-MS-Office365-Filtering-Correlation-Id: a99a5cb6-d5bd-4d06-cd74-08db1445ec12 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:LV2PR12MB5869.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230025)(4636009)(396003)(376002)(39860400002)(136003)(346002)(366004)(451199018)(2906002)(6512007)(6506007)(6486002)(186003)(26005)(38100700002)(7416002)(5660300002)(8936002)(2616005)(83380400001)(41300700001)(86362001)(66476007)(66556008)(4326008)(6916009)(8676002)(66946007)(54906003)(36756003)(316002)(478600001);DIR:OUT;SFP:1101; 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However, it is possible, the APIs allow > > > > it, and for security sandbox purposes we can't allow a process inside > > > > a cgroup to triger a charge on a different cgroup. That breaks the > > > > sandbox goal. > > > > > > It seems broken anyway. Please consider the following scenario: > > > > Yes, this is broken like this already today - memcg doesn't work > > entirely perfectly for MAP_SHARED scenarios, IMHO. > > It is far from perfect but the existing behavior isn't that broken. e.g. in > the same scenario, without pinning, even if the larger cgroup keeps using > the same page, the smaller cgroup should be able to evict the pages as they > are not pinned and the cgroup is under heavy reclaim pressure. The larger > cgroup will refault them back in and end up owning those pages. > > memcg can't capture the case of the same pages being actively shared by > multiple cgroups concurrently (I think those cases should be handled by > pushing them to the common parent as discussed elswhere but that's a > separate topic) but it can converge when page usage transfers across cgroups > if needed. Disassociating ownership and pinning will break that in an > irreversible way. It is already disassociated - memcg is broken as you describe today with pin_user_pages(). If you want to fix that, then we need to redefine how memcg works with pin_user_pages() and I'm open to ideas.. > the owner and pinner disagreeing with each other. At least > conceptually, the solution is rather straight-forward - whoever pins > a page should also claim the ownership of it. If the answer is pinner is owner, then multi-pinners must mean multi-owner too. We probably can't block multi-pinner without causing uAPI problems. You are not wrong on any of these remarks, but this looses sight of the point - it is take the existing broken RLIMIT scheme and make it incrementally better by being the same broken scheme just with cgroups. If we eventually fix everything so memcg can do multi-pinners/owners then would it be reasonable to phase out the new pincg at that time? Jason