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Mon, 17 Feb 2025 11:42:01 +0000 Message-ID: <9c8b047c-e4c9-4abb-88f2-f7e15b59bd9c@intel.com> Date: Mon, 17 Feb 2025 13:40:57 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 1/1] x86/vmemmap: Add missing update of PML4 table / PML5 table entry To: Dave Hansen , "Harry (Hyeonggon) Yoo" <42.hyeyoo@gmail.com> CC: , , , , , , , , References: <20250214195151.168306-1-gwan-gyeong.mun@intel.com> <20250214195151.168306-2-gwan-gyeong.mun@intel.com> <25b9d3f5-bfe8-48a9-b11b-819d19cfae1e@intel.com> <0bed366e-a0e5-4d1a-b677-afad23235bcf@intel.com> Content-Language: en-US From: Gwan-gyeong Mun In-Reply-To: <0bed366e-a0e5-4d1a-b677-afad23235bcf@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: ZR2P278CA0035.CHEP278.PROD.OUTLOOK.COM (2603:10a6:910:47::20) To DS0PR11MB7904.namprd11.prod.outlook.com (2603:10b6:8:f8::8) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB7904:EE_|CO1PR11MB4852:EE_ X-MS-Office365-Filtering-Correlation-Id: d467796d-8a16-4e32-abfe-08dd4f48175f 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|7416014|376014|366016|1800799024; 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Or something that we manage to handle and >>> return from without oopsing? >> >> It means oops, because the kernel accesses part of vmemmap that's not >> populated (yet) in current process's page table. > > Your 0/1 cover letter got to me after this mail did. I see the oops > there clear as day now. > >> This oops was observed after increasing the size of struct page (as a part of >> developing a debug feature), but the real cause is that page table entries are >> only installed in init_mm's page table and then sync'd later, but in the mean >> time the process that triggered hot-plug accesses new portion of vmemmap. >> >> If the process does not directly use the page table of init_mm (like swapper) >> this oops can occur (e.g., I was able to trigger with `sudo modprobe hmm_test` >> after increasing the size of struct page). > > Makes sense. Thanks for the explanation. > >>>> This fixes the problem of using the virtual address without updating the >>>> entry in the PML4 table or PML5 table. But this is a temporary solution to >>>> prevent page fault problems, and it requires improvement of the routine >>>> that updates the missing entry in the PML4 table or PML5 table. >>> >>> Can we please skip past the band-aid and go to the real fix? >> >> Yes, of course it'd best to skip a temporary fix. >> The intention is to report/discuss the problem and a fix as a starting point. > > Do you have a better fix in mind? > Yes, first what comes to mind right now to safely access the virtual address is; translating vmemmap-based virtual address to direct-mapped virtual address and use it, if the current top-level page table is not init_mm's page table when accessing a vmemmap-based virtual address before page table sync. I will send a patch first with this idea. If you have any better ideas, please let me know. Br, G.G.