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emacs 29.1 From: Alistair Popple To: Dan Williams Cc: linux-mm@kvack.org, jhubbard@nvidia.com, rcampbell@nvidia.com, willy@infradead.org, jgg@nvidia.com, david@fromorbit.com, linux-fsdevel@vger.kernel.org, jack@suse.cz, djwong@kernel.org, hch@lst.de, david@redhat.com, ruansy.fnst@fujitsu.com Subject: Re: ZONE_DEVICE refcounting Date: Fri, 22 Mar 2024 11:01:25 +1100 In-reply-to: <65fbcdaf2042f_aa222948c@dwillia2-mobl3.amr.corp.intel.com.notmuch> Message-ID: <874jcz6ryu.fsf@nvdebian.thelocal> Content-Type: text/plain X-ClientProxiedBy: SYYP282CA0004.AUSP282.PROD.OUTLOOK.COM (2603:10c6:10:b4::14) To DS0PR12MB7726.namprd12.prod.outlook.com (2603:10b6:8:130::6) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR12MB7726:EE_|SJ0PR12MB6709:EE_ X-MS-Office365-Filtering-Correlation-Id: d8604ae2-d875-4444-4791-08dc4a055c38 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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AFAICT the >> page->mapping and page->index fields can't actually be used outside of >> fs/dax because they are overloaded for the shared case. Therefore >> setting/clearing them could be skipped and the only reason for doing so >> is so dax_associate_entry()/dax_disassociate_entry() can generate >> warnings which should never occur anyway. So all that code is >> functionally unnecessary. > > What do you mean outside of fs/dax, do you literally mean outside of > fs/dax.c, or the devdax case (i.e. dax without fs-entanglements)? Only the cases fs dax pages might need it. ie. Not devdax which I haven't looked at closely yet. > Memory > failure needs ->mapping and ->index to rmap dax pages. See > mm/memory-failure.c::__add_to_kill() and > mm/memory-failure.c::__add_to_kill_fsdax() where that latter one is for > cases where the fs needs has signed up to react to dax page failure. How does that work for reflink/shared pages which overwrite page->mapping and page->index? Eg. in __add_to_kill() if *p is a shared fs dax page then p->mapping == PAGE_MAPPING_DAX_SHARED and page_address_in_vma(vma, p) will probably crash. Thanks.