On 9/29/26 4:38 PM, David Hildenbrand (Arm) wrote:
On 9/23/26 17:29, Yeoreum Yun wrote:
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.
Reviewed-by: Baolin Wang <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Signed-off-by: Yeoreum Yun <[email protected]>
---
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 2aa7c9197158..b0cb02bf1a73 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -618,6 +618,9 @@ static bool wait_for_scan(const char *msg, char *p, size_t
len,
usleep(TICK);
}
+ if (is_anon(ops))
+ madvise(p, len, MADV_NOHUGEPAGE);
+
Any reason we just do that unconditionally?
Although it's a bit messy, as I mentioned before [1], unconditionally
setting MADV_NOHUGEPAGE will break shmem testing. Maybe add some comments.
[1]
https://lore.kernel.org/all/[email protected]/
