Re: [PATCH v10 3/3] mm: Maintain randomization of page free lists

[Michal Hocko]

On Thu 31-01-19 21:15:27, Dan Williams 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.
> 
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/linux/mmzone.h |   12 ++++++++++++
>  mm/page_alloc.c        |   11 +++++++++--
>  mm/shuffle.c           |   23 +++++++++++++++++++++++
>  mm/shuffle.h           |   12 ++++++++++++
>  4 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 2274e43933ae..a3cb9a21196d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -116,6 +116,18 @@ 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
> +static inline void add_to_free_area_random(struct page *page,
> +		struct free_area *area, int migratetype)
> +{
> +	add_to_free_area(page, area, migratetype);
> +}
> +#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)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3fd0df403766..2a0969e3b0eb 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -43,6 +43,7 @@
>  #include <linux/mempolicy.h>
>  #include <linux/memremap.h>
>  #include <linux/stop_machine.h>
> +#include <linux/random.h>
>  #include <linux/sort.h>
>  #include <linux/pfn.h>
>  #include <linux/backing-dev.h>
> @@ -889,7 +890,8 @@ static inline void __free_one_page(struct page *page,
>  	 * so it's less likely to be used soon and more likely to be merged
>  	 * as a higher order page
>  	 */
> -	if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)) {
> +	if ((order < MAX_ORDER-2) && pfn_valid_within(buddy_pfn)
> +			&& !is_shuffle_order(order)) {
>  		struct page *higher_page, *higher_buddy;
>  		combined_pfn = buddy_pfn & pfn;
>  		higher_page = page + (combined_pfn - pfn);
> @@ -903,7 +905,12 @@ static inline void __free_one_page(struct page *page,
>  		}
>  	}
>  
> -	add_to_free_area(page, &zone->free_area[order], migratetype);
> +	if (is_shuffle_order(order))
> +		add_to_free_area_random(page, &zone->free_area[order],
> +				migratetype);
> +	else
> +		add_to_free_area(page, &zone->free_area[order], migratetype);
> +
>  }
>  
>  /*
> diff --git a/mm/shuffle.c b/mm/shuffle.c
> index 8badf4f0a852..19bbf3e37fb6 100644
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -168,3 +168,26 @@ 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)
> +{
> +	static u64 rand;
> +	static u8 rand_bits;
> +
> +	/*
> +	 * The lack of locking is deliberate. If 2 threads race to
> +	 * update the rand state it just adds to the entropy.
> +	 */
> +	if (rand_bits == 0) {
> +		rand_bits = 64;
> +		rand = get_random_u64();
> +	}
> +
> +	if (rand & 1)
> +		add_to_free_area(page, area, migratetype);
> +	else
> +		add_to_free_area_tail(page, area, migratetype);
> +	rand_bits--;
> +	rand >>= 1;
> +}
> diff --git a/mm/shuffle.h b/mm/shuffle.h
> index 644c8ee97b9e..fc1e327ae22d 100644
> --- a/mm/shuffle.h
> +++ b/mm/shuffle.h
> @@ -36,6 +36,13 @@ static inline void shuffle_zone(struct zone *z)
>  		return;
>  	__shuffle_zone(z);
>  }
> +
> +static inline bool is_shuffle_order(int order)
> +{
> +	if (!static_branch_unlikely(&page_alloc_shuffle_key))
> +                return false;
> +	return order >= SHUFFLE_ORDER;
> +}
>  #else
>  static inline void shuffle_free_memory(pg_data_t *pgdat)
>  {
> @@ -48,5 +55,10 @@ static inline void shuffle_zone(struct zone *z)
>  static inline void page_alloc_shuffle(enum mm_shuffle_ctl ctl)
>  {
>  }
> +
> +static inline bool is_shuffle_order(int order)
> +{
> +	return false;
> +}
>  #endif
>  #endif /* _MM_SHUFFLE_H */
> 

-- 
Michal Hocko
SUSE Labs