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Thu, 5 Jun 2025 01:38:03 +0000 Date: Wed, 4 Jun 2025 18:37:58 -0700 From: Dan Williams To: Alistair Popple , CC: Alistair Popple , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH 02/12] mm: Convert pXd_devmap checks to vma_is_dax Message-ID: <6840f4f65a981_2491100b3@dwillia2-xfh.jf.intel.com.notmuch> References: <224f0265027a9578534586fa1f6ed80270aa24d5.1748500293.git-series.apopple@nvidia.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <224f0265027a9578534586fa1f6ed80270aa24d5.1748500293.git-series.apopple@nvidia.com> X-ClientProxiedBy: BYAPR01CA0025.prod.exchangelabs.com (2603:10b6:a02:80::38) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|IA1PR11MB6242:EE_ X-MS-Office365-Filtering-Correlation-Id: b6e15110-6b18-4268-f4d8-08dda3d19c98 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|7416014|366016; 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Therefore page walkers that want to exclude DAX pages can check > pmd_devmap or pud_devmap. However soon dax will no longer set PFN_DEV, > meaning dax pages are mapped as normal pages. > > Ensure page walkers that currently use pXd_devmap to skip DAX pages > continue to do so by adding explicit checks of the VMA instead. tl;dr: Reviewed-by: Dan Williams So I went through all the p[mu]d_devmap() checks and indeed this is the set I also come up with that are implicitly checking for "dax" instead of checking for "is this a larger than base pte size mapping". While I am a little uncomfortable with the generality of calling the policy "dax" in these locations I think it is ok for now. I.e. the fundamental detail in these paths is "huge pte, but not typical page-allocator THP page" Also I would have felt better if some of the leftover places that are doing "dax" checks but not updated were noted in the changelog just for review purposes. Like: "Note paths like follow_huge_pud and follow_pmd_mask also have 'dax' checks, but those paths are for maintaining dev_pagemap refcounts which no longer (since v6.15) need to be managed for dax pages. A later patch cleans those up."