Re: [PATCH v2 3/5] mm: add static PMD zero page

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

On Thu, Jul 17, 2025 at 01:46:03PM +0200, David Hildenbrand wrote:
> On 17.07.25 12:34, Pankaj Raghav (Samsung) wrote:
> > > > Then, we'd only need a config option to allow for that to happen.
> > > 
> > > Something incomplete and very hacky just to give an idea. It would try allocating
> > > it if there is actual code running that would need it, and then have it
> > > stick around forever.
> > > 
> > Thanks a lot for this David :) I think this is a much better idea and
> > reduces the amount code and reuse the existing infrastructure.
> > 
> > I will try this approach in the next version.
> > 
> > <snip>
> > > +       /*
> > > +        * Our raised reference will prevent the shrinker from ever having
> > > +        * success -> static.
> > > +        */
> > > +       if (atomic_read(&huge_zero_folio_is_static))
> > > +               return huge_zero_folio;
> > > +       /* TODO: memblock allocation if buddy is not up yet? Or Reject that earlier. */
> > 
> > Do we need memblock allocation? At least the use cases I forsee for
> > static pmd zero page are all after the mm is up. So I don't see why we
> > need to allocate it via memblock.
> 
> Even better!
> 
> We might want to detect whether allocation of the huge zeropage failed a
> couple of times and then just give up. Otherwise, each and every user of the
> largest zero folio will keep allocating it.

Yes, that makes sense. We need sort of like a global counter to count
the nr of failures and then give up trying to allocate it if it goes
above a threshold.

--
Pankaj