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/ > > 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 <[email protected]> > --- > 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 <[email protected]> Best Regards, Yan, Zi
