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

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Wed, Sep 23, 2026 at 11:28:33AM +0800, Baolin Wang 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@arm.com>
> > ---
> >   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.

Yeap. That would be much clear.

> 
> With that, LGTM.
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Thanks!

-- 
Sincerely,
Yeoreum Yun