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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.1 Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk Content-Language: en-US To: Peter Xu CC: , , Jann Horn , Andrea Arcangeli , James Houghton , Rik van Riel , Miaohe Lin , Nadav Amit , Mike Kravetz , David Hildenbrand , "Andrew Morton" , Muchun Song References: <20221207203034.650899-1-peterx@redhat.com> <20221207203158.651092-1-peterx@redhat.com> <6a970de6-e3da-f57d-14fd-55f65ddcb27d@nvidia.com> From: John Hubbard In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.126.231.37] 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: CO1PEPF00001A63:EE_|CH0PR12MB5251:EE_ X-MS-Office365-Filtering-Correlation-Id: 8c3f4c6a-b149-457b-d109-08dad966d3ae X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Dec 2022 21:54:48.6124 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 8c3f4c6a-b149-457b-d109-08dad966d3ae 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: CO1PEPF00001A63.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH0PR12MB5251 X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: E8B0180011 X-Stat-Signature: qnr99ur6hi7qzeafd3sq1smgcyg7b6ct X-HE-Tag: 1670536493-445499 X-HE-Meta: U2FsdGVkX19tkU6y7WPRcNHp49dK+dh2+V/Zh4pvAQag2R2Qj/75NAqim2wZiSnNAOC9hqGn8d4EjE8/HRb4HtILKTUNq4ihRM8vDmYqsWiq46ZHFjEsj7veqR14sBjqxJSkVYsU25kVRwPVI6rXGQdJDtBoay4su/vIPvwKZvSZQI+M6ifSuVhfLbn0WEMmEMLObVtwss1HCfpP9YZRxt4Lwrmjw6T2J6ySCW8e+ebyDKd04S9JCRNQ1TThJwsBZiSrbR5fzK0vDbrZRjOvm+o6CFIS2UZ6kEF0T9WJwC/tTWOXGnmx2GylVtISKy8HZJTgep//fLelJWOhErt9juXY3efEdyjZbGK4WfjY+4C9JGGKRr2R6NyUMqa43oeCn7Ht2KNyO9qH5rG3js3cP59zWxcLzDTnrWg9LU6wab/0mwibRP73GQGWPUclRK2fmH5qLNyrhPt0apGdJWQTdwoDtec2C9hkrq6CIB7SQadIeppCTp4bw8+6aBh1ticEkNfZzBOT72Ie3yaDaOZA/JtmueElO9OdxNoI3SeYM0L511QM4FmadECxI60FKC1q/3b3B2lkn6laatd2hht7iU2W6Bs6xxo6ZNXq4P5U6ggd5lRyfLqxN6NnkNaPt/jCzOBy8iAWpg2y23H0gfnuNZOlj0xl9t4a5/LdSfa3DT00fT8ZEIQVdxXOFAPwN6hkTbH8tT9jKDlyKPtKUMWW1OaGMiOcvnOPSSmAZiOZkpP3Bvqh7/X4R2QwoRcLct4OCaX8XAmahrsElvEDZtDcIhPPpITJGiYCAUfzuCDpeJfkYfBf0uhS1MtKm49IewpFxpK1o9ftwm5ZMr6kuzK1AB0ey1LQr3jzmPVXIsJ9QLRjzrkb4F4FY6QnuTHb0lr8pJ7yHIWEyQF+aNssAI9tlOLlJ3VhZIAZZqxEXLR+NjzVMEEsmtXJFqAGUlVEt6yc1aLsApYtGmAU7T4lG+x GycOeqjB tdyZGcmkCX77fuomg2kdW1hTKKEDSmB12HTHu7VJd2ghDGGwrJnXn1E9DKr2Ul+671f6tlCZgY912vbMKfqsi7R88tw== 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 12/8/22 13:05, Peter Xu wrote: >>> + /* >>> + * NOTE: we don't need explicit lock here to walk the >>> + * hugetlb pgtable because either (1) potential callers of >>> + * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the >>> + * caller will not walk a hugetlb vma (e.g. ksm or uprobe). >>> + * When one day this rule breaks, one will get a warning >>> + * in hugetlb_walk(), and then we'll figure out what to do. >>> + */ >> >> Confused. Is this documentation actually intended to refer to hugetlb_walk() >> itself, or just this call site? If the former, then let's move it over >> to be right before hugetlb_walk(). > > It is for this specific code path not hugetlb_walk(). > > The "holds i_mmap_rwsem" here is a true statement (not requirement) because > PVMW rmap walkers always have that. That satisfies with hugetlb_walk() > requirements already even without holding the vma lock. > It's really hard to understand. Do you have a few extra words to explain it? I can help with actual comment wording perhaps, but I am still a bit in the dark as to the actual meaning. :) thanks, -- John Hubbard NVIDIA