[PATCH v4 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

From: "Kiryl Shutsemau (Meta)" <[email protected]>

__madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
It clears the global controls only, which is no longer enough.  A per-order
control overrides them, and -s leaves the source order at "always", so
khugepaged collapses the very range the case is working on.  The case then
fails on a collapse that was interfered with rather than refused.

Clear the per-order controls too.  Set them to "inherit", not "never".
khugepaged honours the global never and stays out.  A forced shmem collapse
takes the order it builds from these very controls, and still finds one.

Fixes: 9f0704eae8a4 ("selftests/mm/khugepaged: enlighten for multi-size THP")
Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <[email protected]>
---
 tools/testing/selftests/mm/khugepaged.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 7eb9db0005a0..0008862e7cbc 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -550,8 +550,8 @@ static bool is_anon(struct mem_ops *ops)
 static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
 			       struct mem_ops *ops, bool expect)
 {
-	int ret;
 	struct thp_settings settings = *thp_current_settings();
+	int ret, i;
 
 	ksft_print_msg("%s...", msg);
 
@@ -564,9 +564,20 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
 	/*
 	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
 	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
+	 *
+	 * The per-order controls have to go too, not just the global one: a
+	 * source order left at "always" -- which -s does -- lets khugepaged
+	 * collapse the very range the case is working on.  Set them to
+	 * "inherit", not "never".  khugepaged honours the global never and
+	 * stays out.  A forced shmem collapse takes the order it builds from
+	 * these very controls, and still finds one.
 	 */
 	settings.thp_enabled = THP_NEVER;
 	settings.shmem_enabled = SHMEM_NEVER;
+	for (i = 0; i < NR_ORDERS; i++) {
+		settings.hugepages[i].enabled = THP_INHERIT;
+		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
+	}
 	thp_push_settings(&settings);
 
 	/* Clear VM_NOHUGEPAGE */
-- 
2.54.0