Re: [PATCH v7 3/3] mm: Maintain randomization of page free lists

[Michal Hocko]

On Mon 07-01-19 15:21:21, 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: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  include/linux/mmzone.h  |   10 ++++++++++
>  include/linux/shuffle.h |   12 ++++++++++++
>  mm/page_alloc.c         |   11 +++++++++--
>  mm/shuffle.c            |   16 ++++++++++++++++
>  4 files changed, 47 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index b78a45e0b11c..c15f7f703be0 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -98,6 +98,8 @@ extern int page_group_by_mobility_disabled;
>  struct free_area {
>  	struct list_head	free_list[MIGRATE_TYPES];
>  	unsigned long		nr_free;
> +	u64			rand;
> +	u8			rand_bits;
>  };

Do we really need per order randomness? Why a global one is not
sufficient?
-- 
Michal Hocko
SUSE Labs