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Tue, 2 Mar 2021 00:06:01 +0000 Date: Mon, 1 Mar 2021 20:05:59 -0400 From: Jason Gunthorpe To: Alistair Popple CC: , , , , , , , , , , , Subject: Re: [PATCH v3 5/8] mm: Device exclusive memory access Message-ID: <20210302000559.GA763995@nvidia.com> References: <20210226071832.31547-1-apopple@nvidia.com> <20210226071832.31547-6-apopple@nvidia.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20210226071832.31547-6-apopple@nvidia.com> X-ClientProxiedBy: BLAPR03CA0058.namprd03.prod.outlook.com (2603:10b6:208:32d::33) To DM6PR12MB3834.namprd12.prod.outlook.com (2603:10b6:5:14a::12) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from mlx.ziepe.ca (142.162.115.133) by BLAPR03CA0058.namprd03.prod.outlook.com (2603:10b6:208:32d::33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3890.19 via Frontend Transport; Tue, 2 Mar 2021 00:06:00 +0000 Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1lGsY3-003Clj-EX; 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envelope-from=""; helo=hqnvemgate25.nvidia.com; client-ip=216.228.121.64 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614643568-667183 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: On Fri, Feb 26, 2021 at 06:18:29PM +1100, Alistair Popple wrote: > +/** > + * make_device_exclusive_range() - Mark a range for exclusive use by a device > + * @mm: mm_struct of assoicated target process > + * @start: start of the region to mark for exclusive device access > + * @end: end address of region > + * @pages: returns the pages which were successfully mark for exclusive acces > + * > + * Returns: number of pages successfully marked for exclusive access > + * > + * This function finds the ptes mapping page(s) to the given address range and > + * replaces them with special swap entries preventing userspace CPU access. On > + * fault these entries are replaced with the original mapping after calling MMU > + * notifiers. > + */ > +int make_device_exclusive_range(struct mm_struct *mm, unsigned long start, > + unsigned long end, struct page **pages) > +{ > + long npages = (end - start) >> PAGE_SHIFT; > + long i; > + > + npages = get_user_pages_remote(mm, start, npages, > + FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD, > + pages, NULL, NULL); > + for (i = 0; i < npages; i++) { > + if (!trylock_page(pages[i])) { > + put_page(pages[i]); > + pages[i] = NULL; > + continue; > + } > + > + if (!try_to_protect(pages[i])) { Isn't this racy? get_user_pages returns the ptes at an instant in time, they could have already been changed to something else? I would think you'd want to switch to the swap entry atomically under th PTLs? Jason