Re: [PATCH v4 03/13] mm/vmemmap: Improve vmemmap_can_optimize and allow architectures to override

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 10/07/2023 à 18:08, Aneesh Kumar K.V a écrit :
> dax vmemmap optimization requires a minimum of 2 PAGE_SIZE area within
> vmemmap such that tail page mapping can point to the second PAGE_SIZE area.
> Enforce that in vmemmap_can_optimize() function.
> 
> Architectures like powerpc also want to enable vmemmap optimization
> conditionally (only with radix MMU translation). Hence allow architecture
> override.
> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

Why renaming vmemmap_can_optimize() to __vmemmap_can_optimize() and keep 
it when vmemmap_can_optimize() has been override ? Is that because you 
expect overriding version of vmemmap_can_optimize() to call 
__vmemmap_can_optimize() ?

> ---
>   include/linux/mm.h | 27 +++++++++++++++++++++++----
>   mm/mm_init.c       |  2 +-
>   2 files changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2dd73e4f3d8e..1a2234ee14d2 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3639,13 +3639,32 @@ void vmemmap_free(unsigned long start, unsigned long end,
>   		struct vmem_altmap *altmap);
>   #endif
>   
> +#define VMEMMAP_RESERVE_NR	2
>   #ifdef CONFIG_ARCH_WANT_OPTIMIZE_VMEMMAP
> -static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
> -					   struct dev_pagemap *pgmap)
> +static inline bool __vmemmap_can_optimize(struct vmem_altmap *altmap,
> +					  struct dev_pagemap *pgmap)
>   {
> -	return is_power_of_2(sizeof(struct page)) &&
> -		pgmap && (pgmap_vmemmap_nr(pgmap) > 1) && !altmap;
> +	unsigned long nr_pages;
> +	unsigned long nr_vmemmap_pages;
> +
> +	if (!pgmap || !is_power_of_2(sizeof(struct page)))
> +		return false;
> +
> +	nr_pages = pgmap_vmemmap_nr(pgmap);
> +	nr_vmemmap_pages = ((nr_pages * sizeof(struct page)) >> PAGE_SHIFT);
> +	/*
> +	 * For vmemmap optimization with DAX we need minimum 2 vmemmap
> +	 * pages. See layout diagram in Documentation/mm/vmemmap_dedup.rst
> +	 */
> +	return !altmap && (nr_vmemmap_pages > VMEMMAP_RESERVE_NR);
>   }
> +/*
> + * If we don't have an architecture override, use the generic rule
> + */
> +#ifndef vmemmap_can_optimize
> +#define vmemmap_can_optimize __vmemmap_can_optimize
> +#endif
> +
>   #else
>   static inline bool vmemmap_can_optimize(struct vmem_altmap *altmap,
>   					   struct dev_pagemap *pgmap)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index a1963c3322af..245ac69b66a5 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1020,7 +1020,7 @@ static inline unsigned long compound_nr_pages(struct vmem_altmap *altmap,
>   	if (!vmemmap_can_optimize(altmap, pgmap))
>   		return pgmap_vmemmap_nr(pgmap);
>   
> -	return 2 * (PAGE_SIZE / sizeof(struct page));
> +	return VMEMMAP_RESERVE_NR * (PAGE_SIZE / sizeof(struct page));
>   }
>   
>   static void __ref memmap_init_compound(struct page *head,