Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm_slot()

[Lance Yang]

On 2025/9/16 14:42, Hugh Dickins wrote:
> On Tue, 16 Sep 2025, Lance Yang wrote:
>
> > Hi Hugh,
> >
> > Thanks for taking a look and for raising this important point!
> >
> > On 2025/9/16 13:32, Hugh Dickins wrote:
> > > On Sun, 14 Sep 2025, Lance Yang wrote:
> > >
> > > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > > >
> > > > Let's skip unsuitable VMAs early in the khugepaged scan; specifically,
> > > > mlocked VMAs should not be touched.
> > >
> > > Why?  That's a change in behaviour, isn't it?
> > >
> > > I'm aware that hugepage collapse on an mlocked VMA can insert a fault
> > > latency, not universally welcome; but I've not seen discussion, let
> > > alone agreement, that current behaviour should be changed.
> > > Somewhere in yet-to-be-read mail?  Please give us a link.
> > >
> > > Hugh
> >
> > You're right, this is indeed a change in behaviour. But it's specifically
> > for khugepaged.
> >
> > Users of mlock() expect low and predictable latency. THP collapse is a
> > heavy operation that introduces exactly the kind of unpredictable delays
> > they want to avoid. It has to unmap PTEs, copy data from the small folios
> > to a new THP, and then remap the THP back to the PMD ;)
> >
> > IMO, that change is acceptable because THP is generally transparent to
> > users, and khugepaged does not guarantee when THP collapse or split will
> > happen.
>
> I disagree.  Many of those who have khugepaged enabled would prefer
> it to give them hugepages, even or especially on mlocked areas.
>
> If you make that change, it must be guarded by a sysfs or sysctl tuning.

Thanks for the feedback!

Well, seems like we're not on the same page. Let's gather more opinions from
other folks ;)

> Perhaps it could share the sysctl_compact_unevictable_allowed tuning
> (I'm not sure whether that's a good or bad idea: opinions will differ).

Thanks,
Lance

> Hugh
>
> > Well, we don't have a discussion on that, just something I noticed.
> >
> > Thanks,
> > Lance