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McKenney" , John Dias , David Hildenbrand References: <20220510211743.95831-1-minchan@kernel.org> <857d21da-5de2-fa3e-b1ce-41cc1cfb0191@nvidia.com> From: John Hubbard In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR13CA0033.namprd13.prod.outlook.com (2603:10b6:a03:2c2::8) To BY5PR12MB4130.namprd12.prod.outlook.com (2603:10b6:a03:20b::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 54ea1d2e-e3a1-43c9-ff45-08da32e0f2fd X-MS-TrafficTypeDiagnostic: IA1PR12MB6114:EE_ X-Microsoft-Antispam-PRVS: 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:BY5PR12MB4130.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230001)(4636009)(366004)(6512007)(26005)(2616005)(186003)(66946007)(31686004)(8936002)(66556008)(4326008)(66476007)(8676002)(36756003)(5660300002)(316002)(54906003)(2906002)(6916009)(508600001)(38100700002)(6506007)(6486002)(31696002)(53546011)(86362001)(83380400001)(43740500002)(45980500001);DIR:OUT;SFP:1101; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=nh4qcMTI; spf=none (imf11.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.220.88) smtp.mailfrom=jhubbard@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: A7C0B40099 X-Stat-Signature: r47k41dixh93yu8wxxpxyo3ic69icp17 X-HE-Tag: 1652227093-40127 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 5/10/22 4:31 PM, Minchan Kim wrote: >>> + int __mt = get_pageblock_migratetype(page); >>> + int mt = __READ_ONCE(__mt); >> >> Although I saw the email discussion about this in v2, that discussion >> didn't go far enough. It started with "don't use volatile", and went >> on to "try __READ_ONCE() instead", but it should have continued on >> to "you don't need this at all". > > That's really what I want to hear from experts so wanted to learn > "Why". How could we prevent refetching of the mt if we don't use > __READ_ONCE or volatile there? > >> >> Because you don't. There is nothing you are racing with, and adding >> __READ_ONCE() in order to avoid a completely not-going-to-happen >> compiler re-invocation of a significant code block is just very wrong. >> >> So let's just let it go entirely. :) > > Yeah, once it's clear for everyone, I am happy to remove the > unnecessary lines. > >> >>> + >>> + if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE) >> With or without __READ_ONCE() or volatile or anything else, this code will do what you want. Which is: loosely check for either of the above. What functional problem do you think you are preventing with __READ_ONCE()? Because I don't see one. thanks, -- John Hubbard NVIDIA