Re: [PATCH v2 07/20] hugetlb_vmemmap: Disable HVO sysctl if arch doesn't support HVO

[James Houghton]

On Fri, Nov 6, 2026 at 11:02 PM Muchun Song <muchun.song@xxxxxxxxx> wrote:
>
>
>
> > On Oct 3, 2026, at 08:21, James Houghton <jthoughton@xxxxxxxxxx> wrote:
> >
> > Do not create the vm.hugetlb_optimize_vmemmap sysctl when the
> > architecture lacks support. This serves only to avoid confusion for
> > users.
> >
> > arm64 support for HVO is conditional on certain hardware features being
> > enabled for all online CPUs. By the time hugetlb_vmemmap_init() runs,
> > all secondary CPUs will have started, and the system feature set will
> > have stabilized, including HVO-through-BBM, which
> > arch_hugetlb_vmemmap_optimization_supported() checks for.
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> > mm/hugetlb_vmemmap.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 8596a462fee6..817ce39d4cf3 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -763,6 +763,21 @@ static int __init hugetlb_vmemmap_init(void)
> >       /* HUGETLB_VMEMMAP_RESERVE_SIZE should cover all used struct pages */
> >       BUILD_BUG_ON(__NR_USED_SUBPAGE > HUGETLB_VMEMMAP_RESERVE_PAGES);
> >
> > +     /*
> > +      * Architectures that need to probe CPU support for HVO have done
> > +      * such probing now. arch_hugetlb_vmemmap_optimization_supported()
> > +      * will return the correct value.
> > +      */
> > +     if (!arch_hugetlb_vmemmap_optimization_supported()) {
>
> I'm wondering if we could put this architectural check into hugetlb_vmemmap_optimizable(),
>
>
> > +             if (vmemmap_optimize_enabled)
> > +                     pr_info("vmemmap optimization not supported by hardware\n");
>
> then, we do not need those code. If the current kernel does not support HVO,
> then the sysctl does not exist. It's already a common case when the size of
> struct page is not power of two. If users really want to know whether HVO
> is enabled, they can check via this sysctl file. Therefore, we don't need to
> print a log line.

Yeah, I can put arch_hugetlb_vmemmap_optimization_supported() into
hugetlb_vmemmap_optimizable(), that will make the diffs slightly
smaller. And yeah I can remove the print. I put it there mostly
because it can be confusing. A command line that gets you HVO on one
arm64 host might not give you HVO on another arm64 host, and the print
would give a user a clue as to why.