Re: [PATCH 1/2] kselftest: mm: prevent random failure of huge page split for khugepaged

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

On Thu, Aug 20, 2026 at 07:37:29PM -0400, Zi Yan wrote:
> On Thu Aug 20, 2026 at 5:23 PM EDT, Yeoreum Yun wrote:
> > On Thu, Aug 20, 2026 at 03:51:34PM -0400, Zi Yan wrote:
> >> On Thu Aug 20, 2026 at 3:40 PM EDT, Yeoreum Yun wrote:
> >> > There're some random failure for split_huge_page_test when khugepaged
> >> > collapses pages into pmd again which had split by the test.
> >> >
> >> > Prevent the khugepaged's collapses for split page by setting the
> >> > mapped pmd-huge-page with MADV_NOHUGEPAGE before split.
> >> 
> >> Why not disable khugepaged using APIs from hugepage_setting.h?
> >
> > Well, this also works with hugepage_save_settings() and write the
> > khugepaged/scan_sleep_millisecs with the maximum integer.
> >
> > I don't have a strong opinion but since it seems more hackish way to
> > change scan_sllep_millisecs, I use madvise() with MADV_NOHUGEPATE.
> >
> > Would it be better to change using APIs in hugepage_setting,h?
> 
> No. I did not realize there is no way of stopping khugepaged completely.
> 
> BTW, I am curious about your khugepaged config causing this issue.
> IIRC, khugepaged scan interval is about minutes and
> the time between madvise(MADV_HUGEPAGE) and the second
> is_backed_by_folio() should not be that long.

Yes. it's relly timing sensitive and I've failed to reproduce in the
real hardware. but in the emulation environment it often failed with 
the scan_sllep_millisecs as 10 seconds while the test running.

> 
> For your patch, maybe you want to check the return value of madvise() to
> make sure it succeeds.

Yeap. but it seems enough just worning message for the madvise() with
MADV_NOHUGEPAGE.

> 
> Otherwise,
> 
> feel free to add
> 
> Reviewed-by: Zi Yan <ziy@nvidia.com>

Thanks!

-- 
Sincerely,
Yeoreum Yun