On Fri, 19 Mar 2021, Johannes Weiner wrote:
> Since commit 2d1c498072de ("mm: memcontrol: make swap tracking an
> integral part of memory control"), the cgroup swap arrays are used to
> track memory ownership at the time of swap readahead and swapoff, even
> if swap space *accounting* has been turned off by the user via
> swapaccount=0 (which sets cgroup_memory_noswap).
>
> However, the patch was overzealous: by simply dropping the
> cgroup_memory_noswap conditionals in the swapon, swapoff and uncharge
> path, it caused the cgroup arrays being allocated even when the memory
> controller as a whole is disabled. This is a waste of that memory.
>
> Restore mem_cgroup_disabled() checks, implied previously by
> cgroup_memory_noswap, in the swapon, swapoff, and swap_entry_free
> callbacks.
>
> Fixes: 2d1c498072de ("mm: memcontrol: make swap tracking an integral part of memory control")
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Thanks for the memory!
> ---
> mm/memcontrol.c | 3 +++
> mm/swap_cgroup.c | 6 ++++++
> 2 files changed, 9 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 668d1d7c2645..49bdcf603af1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7101,6 +7101,9 @@ void mem_cgroup_uncharge_swap(swp_entry_t entry, unsigned int nr_pages)
> struct mem_cgroup *memcg;
> unsigned short id;
>
> + if (mem_cgroup_disabled())
> + return;
> +
> id = swap_cgroup_record(entry, 0, nr_pages);
> rcu_read_lock();
> memcg = mem_cgroup_from_id(id);
> diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
> index 7f34343c075a..08c3246f9269 100644
> --- a/mm/swap_cgroup.c
> +++ b/mm/swap_cgroup.c
> @@ -171,6 +171,9 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
> unsigned long length;
> struct swap_cgroup_ctrl *ctrl;
>
> + if (mem_cgroup_disabled())
> + return 0;
> +
> length = DIV_ROUND_UP(max_pages, SC_PER_PAGE);
> array_size = length * sizeof(void *);
>
> @@ -206,6 +209,9 @@ void swap_cgroup_swapoff(int type)
> unsigned long i, length;
> struct swap_cgroup_ctrl *ctrl;
>
> + if (mem_cgroup_disabled())
> + return;
> +
> mutex_lock(&swap_cgroup_mutex);
> ctrl = &swap_cgroup_ctrl[type];
> map = ctrl->map;
> --
> 2.30.1
