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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.11.2 Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan Content-Language: en-US To: Matthew Wilcox CC: David Hildenbrand , Khalid Aziz , Steven Sistare , , , , , , , Khalid Aziz References: <60367660-f4a3-06dc-4d17-4dbdc733ef74@oracle.com> <846b770c-9f63-90a2-0435-ec82484e3f74@nvidia.com> From: John Hubbard In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.126.231.35] X-ClientProxiedBy: rnnvmail201.nvidia.com (10.129.68.8) To rnnvmail201.nvidia.com (10.129.68.8) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: CY4PEPF0000E9D2:EE_|CH2PR12MB4972:EE_ X-MS-Office365-Filtering-Correlation-Id: cbb9956d-6a37-41f6-64d3-08db5fd64076 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:216.228.117.161;CTRY:US;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:mail.nvidia.com;PTR:dc6edge2.nvidia.com;CAT:NONE;SFS:(13230028)(4636009)(39860400002)(346002)(396003)(136003)(376002)(451199021)(46966006)(40470700004)(36840700001)(36860700001)(40460700003)(83380400001)(47076005)(5660300002)(316002)(70206006)(70586007)(36756003)(7416002)(6916009)(4326008)(82310400005)(82740400003)(31696002)(356005)(8676002)(7636003)(41300700001)(8936002)(86362001)(40480700001)(54906003)(53546011)(2906002)(2616005)(16576012)(186003)(31686004)(16526019)(336012)(426003)(478600001)(26005)(43740500002);DIR:OUT;SFP:1101; X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 28 May 2023 23:50:01.1104 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: cbb9956d-6a37-41f6-64d3-08db5fd64076 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CY4PEPF0000E9D2.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH2PR12MB4972 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/26/23 20:18, Matthew Wilcox wrote: > On Fri, May 26, 2023 at 07:11:05PM -0700, John Hubbard wrote: >>> So any user with 1024 processes can fragment physical memory? :/ >>> >>> Sorry, I'd like to minimize the usage of folio_maybe_dma_pinned(). >> >> I was actually thinking that we should minimize any more cases of >> fragile mapcount and refcount comparison, which then leads to >> Matthew's approach here! > > I was wondering if we shouldn't make folio_maybe_dma_pinned() a little > more accurate. eg: > > if (folio_test_large(folio)) > return atomic_read(&folio->_pincount) > 0; > return (unsigned)(folio_ref_count(folio) - folio_mapcount(folio)) >= > GUP_PIN_COUNTING_BIAS; I'm trying to figure out what might be wrong with that, but it seems OK. We must have talked about this earlier, but I recall vaguely that there was not a lot of concern about the case of a page being mapped > 1024 times. Because pinned or not, it's likely to be effectively locked into memory due to LRU effects. As mentioned here, too. Anyway, sure. A detail: The unsigned cast, I'm not sure that helps or solves anything, right? That is, other than bugs, is it possible to get refcount < mapcount? And if it's only due to bugs, then the casting, again, isn't likely to going to mitigate the fallout from whatever mess the bug caused. thanks, -- John Hubbard NVIDIA