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David Alan Gilbert" , Peter Xu , Markus Armbruster , Paolo Bonzini , Den Lunev References: <20210121152458.193248-1-andrey.gruzdev@virtuozzo.com> From: Andrey Gruzdev Message-ID: <5d01402e-273a-53cf-b78b-b4b7f50340bc@virtuozzo.com> Date: Tue, 9 Feb 2021 21:38:07 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [95.165.26.68] X-ClientProxiedBy: AM4PR0902CA0020.eurprd09.prod.outlook.com (2603:10a6:200:9b::30) To AM0PR08MB3364.eurprd08.prod.outlook.com (2603:10a6:208:e4::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.64] (95.165.26.68) by AM4PR0902CA0020.eurprd09.prod.outlook.com (2603:10a6:200:9b::30) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3825.20 via Frontend Transport; Tue, 9 Feb 2021 18:38:08 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: b69630a6-829f-4712-e9fd-08d8cd29d92f X-MS-TrafficTypeDiagnostic: AM9PR08MB6131: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:9508; 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envelope-from=andrey.gruzdev@virtuozzo.com; helo=EUR03-DB5-obe.outbound.protection.outlook.com X-Spam_score_int: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.265, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 09.02.2021 15:37, David Hildenbrand wrote: > On 21.01.21 16:24, andrey.gruzdev--- via wrote: >> This patch series is a kind of 'rethinking' of Denis Plotnikov's >> ideas he's >> implemented in his series '[PATCH v0 0/4] migration: add background >> snapshot'. >> >> Currently the only way to make (external) live VM snapshot is using >> existing >> dirty page logging migration mechanism. The main problem is that it >> tends to >> produce a lot of page duplicates while running VM goes on updating >> already >> saved pages. That leads to the fact that vmstate image size is >> commonly several >> times bigger then non-zero part of virtual machine's RSS. Time >> required to >> converge RAM migration and the size of snapshot image severely depend >> on the >> guest memory write rate, sometimes resulting in unacceptably long >> snapshot >> creation time and huge image size. >> >> This series propose a way to solve the aforementioned problems. This >> is done >> by using different RAM migration mechanism based on UFFD write >> protection >> management introduced in v5.7 kernel. The migration strategy is to >> 'freeze' >> guest RAM content using write-protection and iteratively release >> protection >> for memory ranges that have already been saved to the migration stream. >> At the same time we read in pending UFFD write fault events and save >> those >> pages out-of-order with higher priority. >> > > Hi, > > just stumbled over this, quick question: > > I recently played with UFFD_WP and notices that write protection is > only effective on pages/ranges that have already pages populated (IOW: > !pte_none() in the kernel). > > In case memory was never populated (or was discarded using e.g., > madvice(DONTNEED)), write-protection will be skipped silently and you > won't get WP events for applicable pages. > > So if someone writes to a yet unpoupulated page ("zero"), you won't > get WP events. > > I can spot that you do a single uffd_change_protection() on the whole > RAMBlock. > > How are you handling that scenario, or why don't you have to handle > that scenario? > Hi David, I really wonder if such a problem exists.. If we are talking about a write to an unpopulated page, we should get first page fault on non-present page and populate it with protection bits from respective vma. For UFFD_WP vma's  page will be populated non-writable. So we'll get another page fault on present but read-only page and go to handle_userfault. -- Andrey Gruzdev, Principal Engineer Virtuozzo GmbH +7-903-247-6397 virtuzzo.com