Re: [PATCH v8 03/14] mm: introduce pgtable_has_pmd_leaves()

[Zi Yan]

On Thu Sep 17, 2026 at 9:45 PM EDT, Luiz Capitulino wrote:
> Currently, we have two helpers that check for PMD-sized pages but have
> different names and slightly different semantics:
>
> - has_transparent_hugepage(): the name suggests it checks if THP is
>   enabled, but when CONFIG_TRANSPARENT_HUGEPAGE=y and the architecture
>   implements this helper, it actually checks if the CPU supports
>   PMD-sized pages
>
> - thp_disabled_by_hw(): the name suggests it checks if THP is disabled
>   by the hardware, but it just returns a cached value acquired with
>   has_transparent_hugepage() during boot. This helper is used in fast
>   paths
>
> A better design would be to separate CONFIG_TRANSPARENT_HUGEPAGE
> checking, which can be done with the IS_ENABLED() macro, from checking
> if a CPU supports PMD-sized pages.
>
> To this end, this commit introduces a new helper called
> pgtable_has_pmd_leaves() which offers the following advantages:
>
>  1. Well defined and clear semantics: it returns true if the CPU
>     supports PMD-sized pages and false otherwise
>  2. It always returns a cached value, so it can be used in fast paths
>  3. It's implemented with a static key: it's a no-op for archs not
>     implemeting it and for archs implementing it the static key is
>     changed only during boot
>
> The new helper requires an initialization step which is performed by
> pgtable_leaf_support_init(). We call pgtable_leaf_support_init() early
> during boot from mm_core_init().
>
> The next commits will convert users of both has_transparent_hugepage()
> and thp_disabled_by_hw() to pgtable_has_pmd_leaves() and/or
> IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE).
>
> Signed-off-by: Luiz Capitulino <luizcap@xxxxxxxxxx>
> ---
>  include/linux/pgtable.h | 17 +++++++++++++++++
>  mm/memory.c             |  9 +++++++++
>  mm/mm_init.c            |  1 +
>  3 files changed, 27 insertions(+)
>
LGTM.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>


-- 
Best Regards,
Yan, Zi