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b=OcMGJfROeAkAi1TOw9J5rtJAw0pKovKAG4Ldl+hW/+sY1kFAGs8x3yWre4Lvy4neS Muvg76OrE4AlupG60VCKZV1KV/8SVY5OiABXbDlA4Xn5sFlS2XVSAymAP47IAJpICq wEdky5z2ghiwYQJ7rOyMek/zUQo0ew3Wa0sI71R9SZQatTP0OYfqgnLlB8PYH4etu2 jsN3d1b+elZCOdY8Qz1GVjorHwPikLm/LS0QtoznHuqtYNtlpoXkLASxJhNkJJOHAI 2aDORfGNoz2/RvQgbRlzv7SvH+hvEWIw3jMCI5vTMfHnX50072mMTzvGdRV7AfHkCr 6jfv5dHkyo3iQ== Message-ID: Date: Fri, 6 Mar 2026 17:23:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: migrate: requeue destination folio on deferred split queue To: Usama Arif , Zi Yan Cc: Andrew Morton , npache@redhat.com, linux-mm@kvack.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260306133556.2051251-1-usama.arif@linux.dev> <64051a59-680f-40ae-b291-b884aeb7c77b@linux.dev> <993B37FF-29E1-41F5-A1E8-F38B9CD24478@nvidia.com> <28e48b47-f215-4e4a-b55a-01dbf293ff35@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: A254280003 X-Stat-Signature: mxxqhr56oy1wb8osxcgzukchwwrk3pip X-HE-Tag: 1772814243-920557 X-HE-Meta: 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 fklQ93TP 0necWzLv96L53nmxropoi9x/C9wdaedpSUDKhHyq0tjYCOtNWCSiCIKEjnAvgjScZSQgwnylvsEgDBZ96ZKu+dRrxKN88ZP91ijyJRodlq+Mqq+MOm7/m8QWyyL3yD6yd4YIihLOVpyn73TVVd8ubriZGeM+GHVdjRswR37fnYcsVWxduJxONTfLxiOhBAreOV9U2xZF2KxESKh7f/RluDSS11Qs3FUUh/3UV84f9T8J6bCgUhNEEJNRu2MbVDXKkgNSMcUtQBXtPmdiecPw5pO6U+l3/D1PWNgey6flHvySOGSSCBc6igGxvukqx1n0QxijLzOZ9eqNnJeeOcljpnP7OOe2USFMdJ9jlpnBRAmvDtInIweyv8X4cVnlWcqvAjWYO Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/6/26 17:15, Usama Arif wrote: > > > On 06/03/2026 14:46, Zi Yan wrote: >> On 6 Mar 2026, at 9:12, Usama Arif wrote: >> >>> >>> Yes you are right. How about something like below? We also won't need to check >>> for anon and non-device folios with this as we only set the the flag if it was >>> already on deferred_split list. >> >> BTW, migrate_pages() tries to split partially mapped folios before migration[1], >> so what remains in the deferred_list would be: >> >> 1. partially mapped but with a pin, >> 2. fully mapped but potentially underused. >> > > Yes, thats right. > >> I wonder if you want to do an underused scan before migration and try to split >> underused THPs. > > hmm, I think we should keep THPs as is if there is no memory pressure (proactive > or otherwise). Scanning THPs for zeros has a cost and we would also lose the benefit > of THPs when we dont need memory. > >> Or to avoid this additional scan, find a way of detecting >> zero pages at page copy time and split it after migration. >> > > Yeah but I think we lose the benefits of THPs after migration when we dont need > additional memory? > >> Anyway, it seems that all large folios are in this deferred_list. Maybe, like >> David suggested in his LSFMM proposal, we should scan large folios on LRU lists >> at reclaim time instead, since there is not much difference between deferred_list >> and LRU lists right now. >> > > Yeah the THP shrinker is a very basic implementation and there are a lot of > >> >> [1] https://elixir.bootlin.com/linux/v6.19.3/source/mm/migrate.c#L1840 >> > > Also Johannes pointed out its not great storing this information in page flags, > we can just keep it as local variable. This is what the patch would look like: Much cleaner. -- Cheers, David