On Fri Aug 7, 2026 at 5:22 PM EDT, Kairui Song wrote: > 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_mcn2e...@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
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. -- Best Regards, Yan, Zi
