On Mon, Oct 19, 2026 at 7:38 AM David Hildenbrand (Arm)
<[email protected]> wrote:
>
> On 10/18/26 06:53, Qiliang Yuan wrote:
> > ksm_tests fills regions with identical data and checks KSM's counters
> > after merging them. allocate_memory() maps these regions without any
> > THP hint.
> >
> > With mTHP enabled the regions can be backed by large folios, which KSM
> > splits before merging. Splitting an anonymous folio maps its
> > zero-filled pages to the shared zeropage, so with use_zero_pages=0
> > those pages never become KSM pages and "KSM zero page merging" fails
> > its count. "ksm_tests -Z -p 10 -z 0", 20 runs:
> >
> > THP setup before after
> > default 0/20 0/20
> > every mTHP size "always" 20/20 0/20
> >
> > Map the regions with MADV_NOHUGEPAGE, as ksm_functional_tests does.
> > ksm_merge_hugepages_time() maps its own THP-backed region and is not
> > affected.
>
>
> That's actually something Nico intends to fix:
>
> https://lore.kernel.org/all/[email protected]/
I will try to get the fix posted today!
>
> >
> > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when
> > splitting isolated thp")
> > Signed-off-by: Qiliang Yuan <[email protected]>
> > ---
> > tools/testing/selftests/mm/ksm_tests.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/tools/testing/selftests/mm/ksm_tests.c
> > b/tools/testing/selftests/mm/ksm_tests.c
> > index 5fd7792a0d479..140ea833353b6 100644
> > --- a/tools/testing/selftests/mm/ksm_tests.c
> > +++ b/tools/testing/selftests/mm/ksm_tests.c
> > @@ -178,6 +178,16 @@ static void *allocate_memory(void *ptr, int prot, int
> > mapping, char data, size_
> > ksft_perror("mmap");
> > return NULL;
> > }
> > + /*
> > + * Splitting a large folio before merging maps its zero-filled pages
> > + * to the shared zeropage, so keep the pages small. Ignore EINVAL on
> > + * kernels without THP.
> > + */
> > + if (madvise(map_ptr, map_size, MADV_NOHUGEPAGE) && errno != EINVAL) {
> > + ksft_perror("madvise");
> > + munmap(map_ptr, map_size);
> > + return NULL;
> > + }
> > memset(map_ptr, data, map_size);
> > if (mprotect(map_ptr, map_size, prot)) {
> > ksft_perror("mprotect");
> >
> > ---
> > base-commit: 90201351a862182bdabfc1ad1c97b1e1b6ed5931
> > change-id: 20261016-bug-selftests-mm-ksm-tests-nohugepage-1414868db0ef
> >
> > Best regards,
>
>
> --
> Cheers,
>
> David
>
