On Mon 06-03-23 23:41:36, Yue Zhao wrote:
> The knob for cgroup v1 memory controller: memory.swappiness
> is not protected by any locking so it can be modified while it is used.
> This is not an actual problem because races are unlikely.
> But it is better to use READ_ONCE/WRITE_ONCE to prevent compiler from
> doing anything funky.
>
> The access of memcg->swappiness and vm_swappiness is lockless,
> so both of them can be concurrently set at the same time
> as we are trying to read them.
>
> Signed-off-by: Yue Zhao <findns94@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> include/linux/swap.h | 8 ++++----
> mm/memcontrol.c | 4 ++--
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 209a425739a9..3f3fe43d1766 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -620,18 +620,18 @@ static inline int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> {
> /* Cgroup2 doesn't have per-cgroup swappiness */
> if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> - return vm_swappiness;
> + return READ_ONCE(vm_swappiness);
>
> /* root ? */
> if (mem_cgroup_disabled() || mem_cgroup_is_root(memcg))
> - return vm_swappiness;
> + return READ_ONCE(vm_swappiness);
>
> - return memcg->swappiness;
> + return READ_ONCE(memcg->swappiness);
> }
> #else
> static inline int mem_cgroup_swappiness(struct mem_cgroup *mem)
> {
> - return vm_swappiness;
> + return READ_ONCE(vm_swappiness);
> }
> #endif
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 06821e5f7604..dca895c66a9b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4179,9 +4179,9 @@ static int mem_cgroup_swappiness_write(struct cgroup_subsys_state *css,
> return -EINVAL;
>
> if (!mem_cgroup_is_root(memcg))
> - memcg->swappiness = val;
> + WRITE_ONCE(memcg->swappiness, val);
> else
> - vm_swappiness = val;
> + WRITE_ONCE(vm_swappiness, val);
>
> return 0;
> }
> --
> 2.17.1
--
Michal Hocko
SUSE Labs
