Re: [stable-6.6.y] mm: khugepaged refuses to freeze

[David Hildenbrand (Arm)]

On 2/6/26 06:12, Baolin Wang wrote:
> On 2/6/26 12:31 PM, Sergey Senozhatsky wrote:
> > On (26/02/06 12:38), Sergey Senozhatsky wrote:
> > [..]
> > > Right, I thought about it but wasn't sure.  Could the inner loop (e.g.
> > > collapse_file() in this particular case) loop long enough to fail 
> > > suspend
> > > w/o ever giving the outer loop (khugepaged_do_scan()) a chance to 
> > > freeze?
>
> Yes, that’s possible. However, if we add a try_to_freeze() check in the 
> inner loop, we need to consider various scenarios (such as anonymous 
> folio swap-in and other potential cases?), which feels too hacky to me.
>
> > For inner loops I wondered if cond_resched() could be an indicator of
> > where try_to_freeze() should be placed.  Those cond_resched() calls
> > are there for a reason, after all.   E.g. something like:
> >
> > ---
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index fa6a018b20a8..cee08466a069 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2431,6 +2431,9 @@ static unsigned int 
> > khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
> >           unsigned long hstart, hend;
> >           cond_resched();
> > +        if (try_to_freeze())
> > +            break;
> > +
> >           if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
> >               progress++;
> >               break;
> > @@ -2453,6 +2456,9 @@ static unsigned int 
> > khugepaged_scan_mm_slot(unsigned int pages, enum scan_result
> >               bool mmap_locked = true;
> >               cond_resched();
> > +            if (try_to_freeze())
> > +                goto breakouterloop;
> > +
> >               if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
> >                   goto breakouterloop;
>
> This looks better than the previous version. Let’s also wait to see if 
> others have any better suggestions.

What prevents other callpaths (faults, read(), write(), etc) from 
similarly triggering swapin?

I recall that there is a notifier when the system is preparing to sleep 
(pm notifier or something). Could we simply hook into that to tell 
khugepaged to suspend+resume?

Essentially, making hpage_collapse_test_exit_or_disable() break our for us.

--
Cheers,

David