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

["Zi Yan" <ziy@nvidia.com>]

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