Re: [PATCH 0/3] mm: support splitting mappingless swapcache folios

[Kairui Song <ryncsn@gmail.com>]

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.