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

On 08/08/26 22:32, Zi Yan wrote:
> On Fri Aug 7, 2026 at 5:22 PM EDT, Kairui Song wrote:
>>
>> 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.
Hi Zi,

I checked the current folio_split() callers and this does not seem
possible.

The shmem swapout path unmaps the folio before clearing folio->mapping,
with the folio locked. The non-uniform split callers either get the folio
from an address_space or from a present page-table entry, so they cannot
find a mappingless shmem folio in swapcache.

The memory-failure path uses split_huge_page_to_order(), which performs a
uniform split.

So I don't think the WARN is reachable with the current callers.

Thanks,
Shivam