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

[Zi Yan]

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@xxxxxxxxxxxxxx/
>
> memory_failure() is one caller affected by the restriction: it cannot
> isolate a poisoned base page within a mappingless swapcache THP. The
> end-to-end test uses this case to exercise the complete shmem pageout,
> swapcache split, and swapin path.
>
> The first patch contains the MM implementation. The second adds focused
> KUnit coverage for the split eligibility checks. The third adds the
> end-to-end hwpoison selftest, which pages out a shmem THP, poisons a tail
> page, and verifies both isolation and the data read back from swap.
>
> Tests:
>   - The split_hwpoison_swapcache kselftest passes under x86_64 QEMU
>       using virtme-ng 1.41 (2 GiB RAM, 4 KiB pages).
>
> Signed-off-by: Shivam Kalra <shivamkalra98@xxxxxxxxxxx>
> ---
> Shivam Kalra (3):
>       mm/huge_memory: allow splitting mappingless swapcache folios
>       mm: add KUnit coverage for mappingless swapcache folios
>       selftests/mm: test hwpoison recovery of mappingless swapcache THPs
>
>  mm/Kconfig                                         |  14 ++
>  mm/Makefile                                        |   1 +
>  mm/huge_memory.c                                   |  18 +-
>  mm/tests/folio_split_kunit.c                       |  52 ++++
>  tools/testing/selftests/mm/Makefile                |   2 +
>  tools/testing/selftests/mm/run_vmtests.sh          |   1 +
>  .../selftests/mm/split_hwpoison_swapcache.sh       |  57 +++++
>  .../selftests/mm/split_hwpoison_swapcache_test.c   | 261 +++++++++++++++++++++
>  8 files changed, 395 insertions(+), 11 deletions(-)
> ---
> base-commit: 0b53bff4fa05ff0d3ffbd3d3bb10fae69dfab498
> change-id: 20260722-b4-mappingless-swapcache-9badf123138a
>
> Best regards,
> --
> Shivam Kalra <shivamkalra98@xxxxxxxxxxx>


Best Regards,
Yan, Zi