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SubjectRe: [PATCH v9 3/3] mm: Maintain randomization of page free lists
On Tue, 29 Jan 2019 21:02:26 -0800 Dan Williams <dan.j.williams@intel.com> wrote:

> When freeing a page with an order >= shuffle_page_order randomly select
> the front or back of the list for insertion.
>
> While the mm tries to defragment physical pages into huge pages this can
> tend to make the page allocator more predictable over time. Inject the
> front-back randomness to preserve the initial randomness established by
> shuffle_free_memory() when the kernel was booted.
>
> The overhead of this manipulation is constrained by only being applied
> for MAX_ORDER sized pages by default.
>
>
> ...
>
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -98,6 +98,10 @@ extern int page_group_by_mobility_disabled;
> struct free_area {
> struct list_head free_list[MIGRATE_TYPES];
> unsigned long nr_free;
> +#ifdef CONFIG_SHUFFLE_PAGE_ALLOCATOR
> + u64 rand;
> + u8 rand_bits;
> +#endif
> };
>
> /* Used for pages not on another list */
> @@ -116,6 +120,14 @@ static inline void add_to_free_area_tail(struct page *page, struct free_area *ar
> area->nr_free++;
> }
>
> +#ifdef CONFIG_SHUFFLE_PAGE_ALLOCATOR
> +/* Used to preserve page allocation order entropy */
> +void add_to_free_area_random(struct page *page, struct free_area *area,
> + int migratetype);
> +#else
> +#define add_to_free_area_random add_to_free_area

A static inline would be nicer.

> +#endif
> +
> /* Used for pages which are on another list */
> static inline void move_to_free_area(struct page *page, struct free_area *area,
> int migratetype)
>
> ...
>
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -186,3 +186,19 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
> for (z = pgdat->node_zones; z < pgdat->node_zones + MAX_NR_ZONES; z++)
> shuffle_zone(z);
> }
> +
> +void add_to_free_area_random(struct page *page, struct free_area *area,
> + int migratetype)
> +{
> + if (area->rand_bits == 0) {
> + area->rand_bits = 64;
> + area->rand = get_random_u64();
> + }
> +
> + if (area->rand & 1)
> + add_to_free_area(page, area, migratetype);
> + else
> + add_to_free_area_tail(page, area, migratetype);
> + area->rand_bits--;
> + area->rand >>= 1;
> +}

Well that's nice and simple.

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