Re: [PATCH v4 2/9] mm: introduce pgtable_has_pmd_leaves()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/1/26 21:18, 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(). This helper is used in fast paths
> 
> This commit introduces a new helper called pgtable_has_pmd_leaves()
> which is intended to replace both has_transparent_hugepage() and
> thp_disabled_by_hw(). pgtable_has_pmd_leaves() has very clear semantics:
> it returns true if the CPU supports PMD-sized pages and false otherwise.
> It always returns a cached value, so it can be used in fast paths.
> 
> The new helper requires an initialization step which is performed by
> init_arch_has_pmd_leaves(). We call init_arch_has_pmd_leaves() early
> during boot in start_kernel() right after parse_early_param() but before
> parse_args(). This allows early_param() handlers to change CPU flags if
> needed (eg. parse_memopt() in x86-32) while also allowing users to use
> the API from __setup() handlers.
> 
> The next commits will convert users of both has_transparent_hugepage()
> and thp_disabled_by_hw() to pgtable_has_pmd_leaves().
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>  include/linux/pgtable.h | 15 +++++++++++++++
>  init/main.c             |  1 +
>  mm/memory.c             |  9 +++++++++
>  3 files changed, 25 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index cdd68ed3ae1a..b365be3516bf 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -2243,6 +2243,21 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
>  	}
>  }
>  
> +#ifdef CONFIG_MMU
> +DECLARE_STATIC_KEY_TRUE(__arch_has_pmd_leaves_key);
> +static inline bool pgtable_has_pmd_leaves(void)
> +{
> +	return static_branch_likely(&__arch_has_pmd_leaves_key);
> +}
> +void __init init_arch_has_pmd_leaves(void);
> +#else
> +static inline bool pgtable_has_pmd_leaves(void)
> +{
> +	return false;
> +}
> +static inline void __init init_arch_has_pmd_leaves(void) { }
> +#endif
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
> diff --git a/init/main.c b/init/main.c
> index 96f93bb06c49..eea7c5bdddf7 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1053,6 +1053,7 @@ void start_kernel(void)
>  	print_kernel_cmdline(saved_command_line);
>  	/* parameters may set static keys */
>  	parse_early_param();
> +	init_arch_has_pmd_leaves();

Can't we do this a bit later from some mm code?

This feels like something that can just go somewhere into mm_core_init()?

There, we should probably call this something like XXX_init(), and prepare it
from detecting support for PUD leaves as well.

Maybe just

pgtable_init() ?

-- 
Cheers,

David