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Howlett" , Jens Axboe , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, mcgrof@kernel.org, gost.dev@samsung.com, kernel@pankajraghav.com, tytso@mit.edu Subject: Re: [RFC v2 0/3] Decoupling large folios dependency on THP Date: Fri, 27 Feb 2026 15:16:09 -0500 X-Mailer: MailMate (2.0r6290) Message-ID: <4A89F75A-325B-4FFC-AAB6-07C206844B02@nvidia.com> In-Reply-To: <653e34d4-6356-4f9e-8298-40b4e237f761@kernel.org> References: <20251206030858.1418814-1-p.raghav@samsung.com> <8ca84535-861c-4ab0-a46b-5dfe319ac8ac@kernel.org> <653e34d4-6356-4f9e-8298-40b4e237f761@kernel.org> Content-Type: text/plain X-ClientProxiedBy: BY1P220CA0004.NAMP220.PROD.OUTLOOK.COM (2603:10b6:a03:59d::15) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR12MB9473:EE_|SA1PR12MB7409:EE_ X-MS-Office365-Filtering-Correlation-Id: 9d7edebb-9194-4852-e689-08de763d1038 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014; 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This is >> the NOMMU code after all, and the less time we spend on it, the better. >> >>>> So either we need to reimplement all the good stuff that folio_split() >>>> does for us, or we need to make folio_split() available on nommu. >>> >>> folio splitting usually involves unmapping pages, which is rather >>> cumbersome on nommu ;) So we'd have to think about that and the >>> implications. >> >> Depending on your point of view, either everything is mapped on nommu, >> or nothing is mapped ;-) In any case, the folio is freshly-allocated >> and locked, so there's no chance anybody has mapped it yet. >> >>> ramfs_nommu_expand_for_mapping() is all about allocating memory, not >>> splitting something that might already in use somewhere. >>> >>> So I folio_split() on nommu is a bit weird in that context. >> >> Well, it is, but it's also exactly what we need to do -- frees folios >> which are now entirely beyond i_size. And it's code that's also used on >> MMU systems, and the more code that's shared, the better. >> >>> When it comes to allocating memory, I would assume that it would be >>> better (and faster!) to >>> >>> a) allocate a frozen high-order page >>> >>> b) Create the (large) folios directly on chunks of the frozen page, and >>> add them through filemap_add_folio(). >>> >>> We'd have a function that consumes a suitable page range and turns it >>> into a folio (later allocates memdesc). >>> >>> c) Return all unused frozen bits to the page allocator >> >> Right, we could do that. But that's more code and special code in the >> nommu codebase. > > I guess I'd have to see the frankenstein folio_split() to judge if it is > really better :) > > If it's really just about skipping the unmap+remap the end result would > indeed be nice. Without unmap+remap, that is just __folio_freeze_and_split_unmapped()? ;) Best Regards, Yan, Zi