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McKenney" , John Dias , David Hildenbrand References: <20220510211743.95831-1-minchan@kernel.org> <857d21da-5de2-fa3e-b1ce-41cc1cfb0191@nvidia.com> <2ffa7670-04ea-bb28-28f8-93a9b9eea7e8@nvidia.com> From: John Hubbard In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SJ0PR05CA0145.namprd05.prod.outlook.com (2603:10b6:a03:33d::30) 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: 62a30433-48b9-4a8c-f082-08da339d349a X-MS-TrafficTypeDiagnostic: BN7PR12MB2626: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)(86362001)(6512007)(38100700002)(2906002)(2616005)(6916009)(53546011)(31696002)(6506007)(31686004)(36756003)(5660300002)(8936002)(6486002)(4326008)(186003)(66946007)(66476007)(66556008)(8676002)(26005)(316002)(508600001)(83380400001)(54906003)(45980500001)(43740500002);DIR:OUT;SFP:1101; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=Ye8o0yMH; dmarc=pass (policy=reject) header.from=nvidia.com; spf=none (imf26.hostedemail.com: domain of jhubbard@nvidia.com has no SPF policy when checking 40.107.94.53) smtp.mailfrom=jhubbard@nvidia.com X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1652307951-651011 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 5/11/22 2:46 PM, Minchan Kim wrote: >> I read that, but there was never any real justification there for needing >> to prevent a re-read of mt, just a preference: "I'd like to keep use the local >> variable mt's value in folloing conditions checks instead of refetching >> the value from get_pageblock_migratetype." >> >> But I don't believe that there is any combination of values of mt that >> will cause a problem here. >> >> I also think that once we pull in experts, they will tell us that the >> compiler is not going to re-run a non-trivial function to re-fetch a >> value, but I'm not one of those experts, so that's still arguable. But >> imagine what the kernel code would look like if every time we call >> a large function, we have to consider if it actually gets called some >> arbitrary number of times, due to (anti-) optimizations by the compiler. >> This seems like something that is not really happening. > > Maybe, I might be paranoid since I have heard too subtle things > about how compiler could changes high level language code so wanted > be careful especially when we do lockless-stuff. > > Who cares when we change the large(?) function to small(?) function > later on? I'd like to hear from experts to decide it. > Yes. But one thing that is still unanswered, that I think you can answer, is: even if the compiler *did* re-read the mt variable, what problems could that cause? I claim "no problems", because there is no combination of 0, _CMA, _ISOLATE, _CMA|ISOLATE that will cause problems here. Any if that's true, then we can leave the experts alone, because the answer is there without knowing what happens exactly to mt. thanks, -- John Hubbard NVIDIA