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Tue, 4 Feb 2025 19:06:13 +0000 Date: Tue, 4 Feb 2025 11:06:08 -0800 From: Dan Williams To: Alistair Popple , Dan Williams CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH v6 23/26] mm: Remove pXX_devmap callers Message-ID: <67a2652083600_2d2c2942f@dwillia2-xfh.jf.intel.com.notmuch> References: <78cb5dce28a7895cc606e76b8e072efe18e24de1.1736488799.git-series.apopple@nvidia.com> <6786b209929e2_20f32945c@dwillia2-xfh.jf.intel.com.notmuch> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MW4PR03CA0126.namprd03.prod.outlook.com (2603:10b6:303:8c::11) To PH8PR11MB8107.namprd11.prod.outlook.com (2603:10b6:510:256::6) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR11MB8107:EE_|MW3PR11MB4681:EE_ X-MS-Office365-Filtering-Correlation-Id: 65d682e8-fbf2-4655-c95d-08dd454efe29 X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024; 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This tracking was required to ensure the generic mm did not > > > manipulate the page reference counts as FS DAX implemented it's own > > > reference counting scheme. > > > > > > Now that FS DAX pages have their references counted the same way as > > > normal pages this tracking is no longer needed and can be > > > removed. > > > > > > Almost all existing uses of pmd_devmap() are paired with a check of > > > pmd_trans_huge(). As pmd_trans_huge() now returns true for FS DAX pages > > > dropping the check in these cases doesn't change anything. > > > > > > However care needs to be taken because pmd_trans_huge() also checks that > > > a page is not an FS DAX page. This is dealt with either by checking > > > !vma_is_dax() or relying on the fact that the page pointer was obtained > > > from a page list. This is possible because zone device pages cannot > > > appear in any page list due to sharing page->lru with page->pgmap. > > > > While the patch looks straightforward I think part of taking "care" in > > this case is to split it such that any of those careful conversions have > > their own bisect point in the history. > > > > Perhaps this can move to follow-on series to not blow up the patch count > > of the base series? ...but first want to get your reaction to splitting > > for bisect purposes. > > TBH I don't feel too strongly about it - I suppose it would make it easier to > bisect to the specific case we weren't careful enough about. However I think if > a bug is bisected to this particular patch it would be relatively easy based on > the context of the bug to narrow it down to a particular file or two. > > I do however feel strongly about whether or not that should be done in a > follow-on series :-) > > Rebasing such a large series has already become painful and error prone enough > so if we want to split this change up it will definitely need to be a separate > series done once the rest of this has been merged. So I could be pursaded to > roll this and the pfn_t removal (as that depends on devmap going away) together. > > Let me know what you think. I tend to think that there's never any regrets for splitting a patch along lines of risk. I am fine with keeping that in this series if that makes things easier.