Re: [PATCH v11 00/15] khugepaged: mTHP support

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Sep 15, 2025 at 12:52:53PM +0200, David Hildenbrand wrote:
> On 15.09.25 12:48, Lorenzo Stoakes wrote:
> > On Mon, Sep 15, 2025 at 12:44:34PM +0200, David Hildenbrand wrote:
> > > > > > Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy.
> > > > >
> > > > > And at different mTHP levels too right?
> > > > >
> > > >
> > > > Another point here, since we have to keep:
> > > >
> > > > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
> > > >
> > > > Around, and users will try to set values there, presumably we will now add:
> > > >
> > > > /sys/kernel/mm/transparent_hugepage/khugepaged/eagerness
> > > >
> > > > How will we map <-> the two tunables?
> > >
> > > Well, the easy case if someone updates eagerness, then we simply et it to
> > > whatever magic value we compute and document.
> > >
> > > The other direction is more problematic, likely we'll simply warn and do
> > > something reasonable (map it to whatever eagerness scale is closest or
> > > simply indicate it as "-1" -- user intervened or sth like that)
> >
> > I don't love the idea of a -1 situation, as that's going to create some
> > confusion.
>
> swapiness also has a "max" parameter, so we could just say "override" /"
> disabled" / whatever?

I don't love the user being able to override this though, let's just nuke their
ability to set this pleeeease.

Because if they can override it, then we have to do some deeply nasty scaling
for mTHP again.

Would really prefer us to only accept 0/511 + warn on anything else.

We could put the warning in a cycle before we land the change also + just take
Nico's current version for now.

So that way people are aware it's coming...

(Could also put in docs ofc)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo