Re: [PATCH v5 06/19] selftests/mm: stop khugepaged during the MADV_COLLAPSE cases

[Kiryl Shutsemau <kirill@shutemov.name>]

On Wed, Sep 09, 2026 at 05:55:01PM +0800, Baolin Wang wrote:
> 
> 
> On 9/8/26 8:50 PM, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> > 
> > __madvise_collapse() turns THP off before each MADV_COLLAPSE, both to keep
> > khugepaged out of the range and to prove MADV_COLLAPSE ignores the setting.
> > It clears the global controls only, which is no longer enough.  A per-order
> > control overrides them, and -s, which makes the cases fault in folios of
> > one order, leaves that order's control at "always".  khugepaged then
> > collapses the very range the case is working on, and the case fails on a
> > collapse that was interfered with rather than refused.
> 
> Right. So I think the correct fix tag is b7f16963efe7 ("mm/khugepaged: run
> khugepaged for all orders"), because before this commit, khugepaged would
> not try to collapse this range since it only checked whether the PMD order
> was suitable for collapse.

Agreed.  The series is in mm-new already; if a respin is needed I will use
that tag.

> > @@ -547,9 +547,16 @@ static void __madvise_collapse(const char *msg, char *p, int nr_hpages,
> >   	/*
> >   	 * Prevent khugepaged interference and tests that MADV_COLLAPSE
> >   	 * ignores /sys/kernel/mm/transparent_hugepage/enabled
> > +	 *
> > +	 * "inherit" rather than "never" so that MADV_COLLAPSE on shmem still
> > +	 * finds an order to build.
> >   	 */
> >   	settings.thp_enabled = THP_NEVER;
> >   	settings.shmem_enabled = SHMEM_NEVER;
> > +	for (i = 0; i < NR_ORDERS; i++) {
> > +		settings.hugepages[i].enabled = THP_INHERIT;
> > +		settings.shmem_hugepages[i].enabled = SHMEM_INHERIT;
> > +	}
> 
> This looks like a workaround to me. Shouldn't we fix this in shmem instead?

Good point.

It can be a follow-up patch.  Do you want to make a proper shmem.c fix
and update the selftest along with it?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov