Re: [PATCH v2 06/20] hugetlb_vmemmap: Allow architectures to dynamically disallow HVO

[James Houghton]

On Fri, Nov 6, 2026 at 10:44 PM Muchun Song <muchun.song@xxxxxxxxx> wrote:
>
>
>
> > On Oct 3, 2026, at 08:21, James Houghton <jthoughton@xxxxxxxxxx> wrote:
> >
> > For arm64, only some platforms support HVO. Add an arch hook to prevent
> > HVO from taking affect if the architecture lacks support.
> >
> > It may be the case that arch_supports_hugetlb_vmemmap_optimization()
> > returns false at first but eventually returns true. This is the case on
> > arm64. When this happens, bootmem hugepages won't be immediately HVOed,
> > but they will be optimized in prep_and_add_bootmem_folios().
> >
> > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > ---
> > include/asm-generic/hugetlb.h |  7 +++++++
> > mm/hugetlb_vmemmap.c          | 12 ++++++++++++
> > 2 files changed, 19 insertions(+)
> >
> > diff --git a/include/asm-generic/hugetlb.h b/include/asm-generic/hugetlb.h
> > index 635c41cc3479..8d24075d53d4 100644
> > --- a/include/asm-generic/hugetlb.h
> > +++ b/include/asm-generic/hugetlb.h
> > @@ -128,4 +128,11 @@ static inline bool gigantic_page_runtime_supported(void)
> > }
> > #endif /* __HAVE_ARCH_GIGANTIC_PAGE_RUNTIME_SUPPORTED */
> >
> > +#ifndef __HAVE_ARCH_HVO_SUPPORTED
> > +static inline bool arch_hugetlb_vmemmap_optimization_supported(void)
>
> I suggest using arch_support_vmemmap_remap (see [1]), which can also be
> applied to FS-DAX.
>
> [1] https://lore.kernel.org/all/20260903122128.12264-2-songmuchun@xxxxxxxxxxxxx/
>
> > +{
> > +     return IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP);
> > +}
> > +#endif /* __HAVE_ARCH_HVO_SUPPORTED */
> > +
> > #endif /* _ASM_GENERIC_HUGETLB_H */
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 3fdb1e4ce1a1..8596a462fee6 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -16,6 +16,7 @@
> > #include <linux/pagewalk.h>
> > #include <linux/pgalloc.h>
> > #include <linux/vmemmap-optimization.h>
> > +#include <linux/hugetlb.h>
> >
> > #include <asm/tlbflush.h>
> > #include "hugetlb_vmemmap.h"
> > @@ -529,6 +530,9 @@ static bool vmemmap_should_optimize_folio(const struct hstate *h, struct folio *
> >       if (!READ_ONCE(vmemmap_optimize_enabled))
> >               return false;
> >
> > +     if (!arch_hugetlb_vmemmap_optimization_supported())
> > +             return false;
> > +
> >       if (!hugetlb_vmemmap_optimizable(h))
> >               return false;
> >
> > @@ -731,6 +735,14 @@ void __init hugetlb_vmemmap_optimize_bootmem_page(unsigned long pfn, unsigned in
> >       if (!READ_ONCE(vmemmap_optimize_enabled))
> >               return;
> >
> > +     /*
> > +      * Architectures may return false here but true by the time
> > +      * hugetlb_init() is called. In this case, although the folios will
> > +      * not be pre-HVOed, they will be optimized in hugetlb_init().
> > +      */
> > +     if (!arch_hugetlb_vmemmap_optimization_supported())
> > +             return;
>
> I plan to add a new helper, section_vmemmap_optimization_disable() [2],
> intended to disable vmemmap optimization in the architectural vmemmap
> population path. Since arm64 does not support section-based vmemmap
> optimization, section_vmemmap_optimization_disable() should be used there.
>
> [2] https://lore.kernel.org/all/20261107043122.25650-3-songmuchun@xxxxxxxxxxxxx/

Thanks! I'll integrate both of these into v3.