Re: [PATCH v2 2/2] kselftest: mm: fix intermittent failure khugepaged test

[Yeoreum Yun]

> On 9/23/26 12:54, Yeoreum Yun wrote:
> >>
> >>
> >> On 9/21/26 6:35 PM, 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.
> >>>
> >>> 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))
> >>
> >> We usually use the 'if (ops == &__anon_ops)' check in this file to identify
> >> anonymous test cases.
> >>
> >> With that, LGTM.
> >> Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> >> Tested-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > 
> > I think this is merged at mm/mm-unstable already So, I'll send
> > another patch to change it with is_anon().
> 
> Unless it is in mm-stable we can reasonably squash stuff or resend. This
> shouldn't be an independent patch.

I see. Thanks!

-- 
Sincerely,
Yeoreum Yun