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

[Muchun Song <muchun.song@linux.dev>]

> On Oct 3, 2026, at 08:21, James Houghton <jthoughton@google.com> 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@google.com>
> ---
> 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@bytedance.com/

> +{
> + 	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@bytedance.com/

Thanks,
Muchun

> +
> 	section_set_compound_order_range(pfn, 1UL << order, order);
> }
> 
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
>