[PATCH 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Yeoreum Yun]

There are intermittent failures in collapse_max_ptes_swap() and
collapse_max_ptes_shared() when using the khugepaged_context:

  // while running ./khugepaged -s 2

  # Run test: collapse_max_ptes_shared (khugepaged:anon)
  # Allocate huge page... OK
  # Share huge page over fork()... OK
  # Trigger CoW on page 1023 of 2048... OK
  # Maybe collapse with max_ptes_shared exceeded.... OK
  # Trigger CoW on page 1024 of 2048... Fail
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 23)
  # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0

  # Run test: collapse_max_ptes_swap (khugepaged:anon)
  # Swapout 257 of 2048 pages... OK
  # Maybe collapse with max_ptes_swap exceeded.... OK
  # Swapout 256 of 2048 pages... OK
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 17)
  # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0

This happens because khugepaged may collapse the pages before wait_for_scan()
is called, causing a sanity check that expects uncollapsed pages to fail.

For example, in collapse_max_ptes_swap(), after faulting the pages back in
and paging out up to max_ptes_swap pages, khugepaged may collapse them again
before c->collapse() is called.

To prevent this, change the khugepaged setting from ALWAYS to MADVICE for
the affected tests, and mark the VMA with MADV_NOHUGEPAGE after it has been
collapsed by wait_for_scan(). This prevents khugepaged from collapsing it
again before c->collapse() is called.

This failure was observed on NVIDIA Spark with 16KB page.

Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
---
 tools/testing/selftests/mm/khugepaged.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index c32244b565658..83e9386bbc842 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -578,6 +578,8 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
 		usleep(TICK);
 	}
 
+	madvise(p, len, MADV_NOHUGEPAGE);
+
 	return timeout == -1;
 }
 
@@ -839,6 +841,7 @@ static void collapse_swapin_single_pte(struct collapse_context *c, struct mem_op
 
 static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *ops)
 {
+	struct thp_settings settings = *thp_current_settings();
 	int max_ptes_swap = thp_read_num("khugepaged/max_ptes_swap");
 	void *p;
 
@@ -860,6 +863,9 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 	validate_memory(p, 0, hpage_pmd_size);
 
 	if (c->enforce_pte_scan_limits) {
+		settings.hugepages[collapse_order].enabled = THP_MADVISE;
+		thp_push_settings(&settings);
+
 		ops->fault(p, 0, hpage_pmd_size);
 		ksft_print_msg("Swapout %d of %d pages...", max_ptes_swap,
 		       hpage_pmd_nr);
@@ -869,12 +875,15 @@ static void collapse_max_ptes_swap(struct collapse_context *c, struct mem_ops *o
 			success("OK");
 		} else {
 			fail("Fail");
+			thp_pop_settings();
 			goto out;
 		}
 
 		c->collapse("Collapse with max_ptes_swap pages swapped out", p,
 			    1, ops, true);
 		validate_memory(p, 0, hpage_pmd_size);
+
+		thp_pop_settings();
 	}
 out:
 	ops->cleanup_area(p, hpage_pmd_size);
@@ -1075,6 +1084,7 @@ static void collapse_fork_compound(struct collapse_context *c, struct mem_ops *o
 
 static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops *ops)
 {
+	struct thp_settings settings = *thp_current_settings();
 	int max_ptes_shared = thp_read_num("khugepaged/max_ptes_shared");
 	int wstatus;
 	void *p;
@@ -1100,6 +1110,9 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops
 			    1, ops, !c->enforce_pte_scan_limits);
 
 		if (c->enforce_pte_scan_limits) {
+			settings.hugepages[collapse_order].enabled = THP_MADVISE;
+			thp_push_settings(&settings);
+
 			ksft_print_msg("Trigger CoW on page %d of %d...",
 			       hpage_pmd_nr - max_ptes_shared, hpage_pmd_nr);
 			ops->fault(p, 0, (hpage_pmd_nr - max_ptes_shared) *
@@ -1111,6 +1124,8 @@ static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops
 
 			c->collapse("Collapse with max_ptes_shared PTEs shared",
 				    p, 1, ops, true);
+
+			thp_pop_settings();
 		}
 
 		validate_memory(p, 0, hpage_pmd_size);

-- 
2.43.0