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
