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.
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,
--
Qiliang Yuan <[email protected]>
