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Howlett" , "Ryan Roberts" , "Dev Jain" , "Barry Song" , "Lance Yang" , "Usama Arif" , "Vlastimil Babka" , "Mike Rapoport" , "Suren Baghdasaryan" , "Michal Hocko" , "Shuah Khan" , "Nico Pache" , , , To: "Shivam Kalra" , "Kairui Song" From: "Zi Yan" Subject: Re: [PATCH 0/3] mm: support splitting mappingless swapcache folios X-Mailer: aerc 0.21.0 References: <20260805-b4-mappingless-swapcache-v1-0-b1d78ba257ca@zohomail.in> <590B3189-5B66-4ECB-84CD-7B7EF18C0E54@nvidia.com> In-Reply-To: X-ClientProxiedBy: BLAPR03CA0035.namprd03.prod.outlook.com (2603:10b6:208:32d::10) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|IA0PR12MB8088:EE_ X-MS-Office365-Filtering-Correlation-Id: 5c25f074-0626-49de-8260-08def56ee5b2 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|376014|7416014|23010399003|56012099006|22082099003|18002099003|4133799003|4143699003|6133799003|11063799006|10067099003; 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Folios read fr= om swap >> >> > but not yet associated with an anon_vma can have the same mappingle= ss >> >> > swapcache state. folio_check_splittable() currently mistakes both c= ases for >> >> > truncation and rejects the split with -EBUSY. >> >> > >> >> > Implement the longstanding TODO for this state. Allow mappingless >> >> > swapcache folios to use the existing uniform order-0 swapcache spli= t path, >> >> > while continuing to reject truly truncated folios and unsupported >> >> > higher-order or non-uniform swapcache splits. >> >>=20 >> >> Thank you for your patches. >> >>=20 >> >> As Kairui (cc=E2=80=99d) mentioned in [1] (see =E2=80=9CA bit more de= tails on this=E2=80=9D), >> >> we might not need to implement this split. I will let Kairui to decid= e >> >> how we should deal with this patchset. >> >>=20 >> >> [1] https://lore.kernel.org/all/CAMgjq7CHDG8JhesSPMkn1kzG8jKb0BXO756B= vxMQC_MCn2eTyA@mail.gmail.com/ >> > >> > Thanks for the CC, this is actually not hard to implement as shown by >> > Shivam, I just found the code more and more hard to follow and fragile >> > as we add more logic to it. And we don't have to reject higher order >> > split either which can be seen easily if the code is cleaner. >> > >> > Personally I think doing some cleanup first is better, I haven't post = any >> > code as right now there doesn't seem to be much user of this. It will = be >> > needed if more clean THP swapcache begin to show up due to things like >> > THP readahead for swap, which isn't here yet. I think I can send an RF= C >> > tomorrow just for reference. I'm fine if we prefer to remove that TODO >> > using this smaller change first :) >>=20 >> Thanks. Can you also help review this patchset? The changes looks good >> to me but I am not sure if I get all the details about swapcache handlin= g. >>=20 >> --=20 >> Best Regards, >> Yan, Zi > > Hello, > > Thanks for the reminder, I got a bit busy so delayed for a day on that > RFC. > > For this series I think it's functionally fine, just the split logic is > a bit complex to follow and we still have restriction on swap cache > for order 0 uniform split only. I'll have a closer look later. Also I agree. I wonder if it is possible that a shmem in swapcache could enter non uniform split, folio_split(). With Patch 1, folio_check_splittable() will return -EINVAL, causing a WARN. Hi Shivam, Can you check the above issue? If it is possible, we want to avoid it. > let me know how you think about that RFC. Sure, will take a look. --=20 Best Regards, Yan, Zi