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Fri, 07 Aug 2026 14:22:50 -0700 (PDT) Date: Sat, 8 Aug 2026 05:22:41 +0800 From: Kairui Song To: Zi Yan Cc: Shivam Kalra , Kairui Song , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Baolin Wang , "Liam R. Howlett" , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , Nico Pache , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH 0/3] mm: support splitting mappingless swapcache folios Message-ID: References: <20260805-b4-mappingless-swapcache-v1-0-b1d78ba257ca@zohomail.in> <590B3189-5B66-4ECB-84CD-7B7EF18C0E54@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 5FA00C0005 X-Stat-Signature: ap5jgcykqht5oqcus65us6twqkr8n17b X-HE-Tag: 1786137772-679561 X-HE-Meta: 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 um4wLgV0 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 07, 2026 at 10:18:05AM +0800, Zi Yan wrote: > On Wed Aug 5, 2026 at 11:54 AM EDT, Kairui Song wrote: > > On Wed, Aug 05, 2026 at 08:03:33AM +0800, Zi Yan wrote: > >> On 5 Aug 2026, at 7:18, Shivam Kalra via B4 Relay wrote: > >> > >> > Large shmem folios lose their address-space mapping when they are written > >> > to swap, but remain valid members of the swap cache. Folios read from swap > >> > but not yet associated with an anon_vma can have the same mappingless > >> > swapcache state. folio_check_splittable() currently mistakes both cases 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 split path, > >> > while continuing to reject truly truncated folios and unsupported > >> > higher-order or non-uniform swapcache splits. > >> > >> Thank you for your patches. > >> > >> As Kairui (cc’d) mentioned in [1] (see “A bit more details on this”), > >> we might not need to implement this split. I will let Kairui to decide > >> how we should deal with this patchset. > >> > >> [1] https://lore.kernel.org/all/CAMgjq7CHDG8JhesSPMkn1kzG8jKb0BXO756BvxMQC_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 RFC > > tomorrow just for reference. I'm fine if we prefer to remove that TODO > > using this smaller change first :) > > 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 handling. > > -- > 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 let me know how you think about that RFC.