Re: [PATCH 0/2] kselftest: mm: fix intermittent failure khugepaged test

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

On Wed, Sep 16, 2026 at 08:41:00AM +0200, David Hildenbrand (Arm) wrote:
> On 9/15/26 11:21, Yeoreum Yun wrote:
> > There are intermittent failures in collapse_max_ptes_swap() and
> > collapse_max_ptes_shared() when using the khugepaged_context:
> > 
> >   # 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, change the khugepaged setting from ALWAYS to MADVICE for
> > the affected tests, and mark the VMA with MADV_NOHUGEPAGE after it has been
> > collapsed by wait_for_scan(). This prevents khugepaged from collapsing it
> > again before c->collapse() is called.
> 
> ALWAYS also respects MADV_NOHUGEPAGE, so why is the ALWAYS -> MADVICE (MADVISE)
> change required?

You're right. this is redundant and it's enough only set the
VM_NOHUGEPAGE for anon. I'll remove them.

Thanks!

-- 
Sincerely,
Yeoreum Yun