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?
After all, we set the
madvise(p, len, MADV_HUGEPAGE);
unconditionally?
--
Cheers,
David
