Re: [PATCH v3 13/26] mm: introduce freetype_t

> @@ -179,24 +180,62 @@ static inline bool migratetype_is_mergeable(int mt)
>  
>  #define for_each_free_list(list, zone, order) 				\
>  	for (order = 0; order < NR_PAGE_ORDERS; order++) 		\
> -		for (unsigned int __type = 0; 				\
> -		     __type < MIGRATE_TYPES &&				\
> -			(list = &(zone)->free_area[order].free_list[__type], 1); \
> -		     __type++)
> +		for (unsigned int __idx = 0; 				\
> +		     __idx < NR_FREETYPE_IDXS &&			\
> +			(list = &(zone)->free_area[order].free_list[__idx], 1); \
> +		     __idx++)
> +
> +static inline freetype_t migrate_to_freetype(enum migratetype mt,
> +					     unsigned int flags)
> +{
> +	freetype_t freetype;
> +
> +	/* No flags supported yet. */
> +	VM_WARN_ON_ONCE(flags);
> +
> +	freetype.migratetype = mt;
> +	return freetype;
> +}
> +
> +static inline enum migratetype free_to_migratetype(freetype_t freetype)
> +{
> +	return freetype.migratetype;
> +}
> +
> +/* Convenience helper, return the freetype modified to have the migratetype. */
> +static inline freetype_t freetype_with_migrate(freetype_t freetype,
> +					       enum migratetype migratetype)
> +{
> +	return migrate_to_freetype(migratetype, freetype_flags(freetype));
> +}
>  
>  extern int page_group_by_mobility_disabled;
>  
> +freetype_t get_pfnblock_freetype(const struct page *page, unsigned long pfn);
> +
>  #define get_pageblock_migratetype(page) \
>  	get_pfnblock_migratetype(page, page_to_pfn(page))
>  
> +#define get_pageblock_freetype(page) \
> +	get_pfnblock_freetype(page, page_to_pfn(page))
> +
>  #define folio_migratetype(folio) \
>  	get_pageblock_migratetype(&folio->page)
>  
>  struct free_area {
> -	struct list_head	free_list[MIGRATE_TYPES];
> +	struct list_head	free_list[NR_FREETYPE_IDXS];
>  	unsigned long		nr_free;
>  };
>  
> +static inline
> +struct list_head *free_area_list(struct free_area *area, freetype_t type)
> +{
> +	int idx = freetype_idx(type);
> +
> +	VM_WARN_ON(idx < 0);
> +	return &area->free_list[idx];

Should we return NULL here if idx < 0 instead of an out of bounds
access?