Re: [PATCH mm-unstable v17 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Lorenzo Stoakes <ljs@kernel.org>]

On Tue, May 19, 2026 at 01:05:13PM -0600, Nico Pache wrote:
> On Mon, May 18, 2026 at 1:33 PM Lorenzo Stoakes <ljs@kernel.org> wrote:
> >
> > On Mon, May 18, 2026 at 03:16:11PM +0200, David Hildenbrand (Arm) wrote:
> > > > For me, I would vote for fallback to 0.
> > >
> > > At this point I'll prefer to not return errors from collapse_max_ptes_none().
> > > It's just rather awkward to return an error deep down in collapse code for a
> > > configuration problem.
> > >
> > > For mthp collapse, we only support max_ptes_none==0 and
> > > max_ptes_none=="HPAGE_PMD_NR - 1" (default).
> > >
> > > If another value is specified while collapsing mTHP, print a warning and treat
> > > it as 0 (save value, no creep, no memory waste).
> > >
> > > In a sense, this is similar to how we handle max_ptes_shared + max_ptes_swap:
> > > for mTHP: we always treat them as being 0 for mTHP collapse (and don't issue a
> > > warning, because we would issue a warning with the default settings).
> > >
> > > @Lorenzo, fine with you?
> >
> > Yes 100%, this sounds sensible both in terms of the error and the default. Let's
> > keep our lives simple(-ish) please :)
>
> Ok thank you im glad we finally came to consensus on this! phew!
>

It happens sometimes ;)

Cheers, Lorenzo