Re: [PATCH v2] mm: memcontrol: avoid unused function warning

[Michal Hocko]

On Wed 24-08-16 12:23:00, Arnd Bergmann wrote:
> A bugfix in v4.8-rc2 introduced a harmless warning when CONFIG_MEMCG_SWAP
> is disabled but CONFIG_MEMCG is enabled:
> 
> mm/memcontrol.c:4085:27: error: 'mem_cgroup_id_get_online' defined but not used [-Werror=unused-function]
>  static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
> 
> This moves the function inside of the #ifdef block that hides the
> calling function, to avoid the warning.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Fixes: 1f47b61fb407 ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Acked-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

Thanks a lot for respinning this Arnd!

> ---
>  mm/memcontrol.c | 36 ++++++++++++++++++------------------
>  1 file changed, 18 insertions(+), 18 deletions(-)
> 
> This is the alternative to the original patch, as suggested by Michal Hocko.
> Andrew, please pick whichever version you like better.
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2ff0289ad061..9a6a51a7c416 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4082,24 +4082,6 @@ static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n)
>  	atomic_add(n, &memcg->id.ref);
>  }
>  
> -static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
> -{
> -	while (!atomic_inc_not_zero(&memcg->id.ref)) {
> -		/*
> -		 * The root cgroup cannot be destroyed, so it's refcount must
> -		 * always be >= 1.
> -		 */
> -		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
> -			VM_BUG_ON(1);
> -			break;
> -		}
> -		memcg = parent_mem_cgroup(memcg);
> -		if (!memcg)
> -			memcg = root_mem_cgroup;
> -	}
> -	return memcg;
> -}
> -
>  static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n)
>  {
>  	if (atomic_sub_and_test(n, &memcg->id.ref)) {
> @@ -5821,6 +5803,24 @@ static int __init mem_cgroup_init(void)
>  subsys_initcall(mem_cgroup_init);
>  
>  #ifdef CONFIG_MEMCG_SWAP
> +static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
> +{
> +	while (!atomic_inc_not_zero(&memcg->id.ref)) {
> +		/*
> +		 * The root cgroup cannot be destroyed, so it's refcount must
> +		 * always be >= 1.
> +		 */
> +		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
> +			VM_BUG_ON(1);
> +			break;
> +		}
> +		memcg = parent_mem_cgroup(memcg);
> +		if (!memcg)
> +			memcg = root_mem_cgroup;
> +	}
> +	return memcg;
> +}
> +
>  /**
>   * mem_cgroup_swapout - transfer a memsw charge to swap
>   * @page: page whose memsw charge to transfer
> -- 
> 2.9.0

-- 
Michal Hocko
SUSE Labs