[PATCH v2 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, mark the VMA with MADV_NOHUGEPAGE after it has been
collapsed by wait_for_scan() for anon. 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 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index c32244b565658..1aad4bb427ece 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -578,6 +578,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t len,
 		usleep(TICK);
 	}
 
+	if (!strncmp(ops->name, "anon", 4))
+		madvise(p, len, MADV_NOHUGEPAGE);
+
 	return timeout == -1;
 }
 

-- 
2.43.0