Subject: Re: [PATCH memcg v3] net: set proper memcg for net_init hooks allocations

From: Roman Gushchin <[email-masked]>
Date: Mon, 25 Apr 2022 19:50:50 -0700

On Mon, Apr 25, 2022 at 01:56:02PM +0300, Vasily Averin wrote:

Hello, Vasily!

> __register_pernet_operations() executes init hook of registered
> pernet_operation structure in all existing net namespaces.
> 
> Typically, these hooks are called by a process associated with
> the specified net namespace, and all __GFP_ACCOUNTING marked
> allocation are accounted for corresponding container/memcg.

__GFP_ACCOUNT

> 
> However __register_pernet_operations() calls the hooks in the same
> context, and as a result all marked allocations are accounted
> to one memcg for all processed net namespaces.
> 
> This patch adjusts active memcg for each net namespace and helps
> to account memory allocated inside ops_init() into the proper memcg.
> 
> Signed-off-by: Vasily Averin <[email-masked]>
> ---
> v3: put_net_memcg() replaced by an alreay existing mem_cgroup_put()
>     It checks memcg before accessing it, this is required for
>     __register_pernet_operations() called before memcg initialization.
>     Additionally fixed leading whitespaces in non-memcg_kmem version
>     of mem_cgroup_from_obj().
> 
> v2: introduced get/put_net_memcg(),
>     new functions are moved under CONFIG_MEMCG_KMEM
>     to fix compilation issues reported by Intel's kernel test robot
> 
> v1: introduced get_mem_cgroup_from_kmem(), which takes the refcount
>     for the found memcg, suggested by Shakeel
> ---
>  include/linux/memcontrol.h | 29 ++++++++++++++++++++++++++++-
>  net/core/net_namespace.c   |  7 +++++++
>  2 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0abbd685703b..cfb68a3f7015 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1714,6 +1714,29 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
>  
>  struct mem_cgroup *mem_cgroup_from_obj(void *p);
>  
> +static inline struct mem_cgroup *get_mem_cgroup_from_kmem(void *p)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	rcu_read_lock();
> +	do {
> +		memcg = mem_cgroup_from_obj(p);
> +	} while (memcg && !css_tryget(&memcg->css));
> +	rcu_read_unlock();
> +	return memcg;
> +}

Please, rename it to get_mem_cgroup_from_obj() for consistency.

> +
> +static inline struct mem_cgroup *get_net_memcg(void *p)
> +{
> +	struct mem_cgroup *memcg;
> +
> +	memcg = get_mem_cgroup_from_kmem(p);
> +
> +	if (!memcg)
> +		memcg = root_mem_cgroup;
> +
> +	return memcg;
> +}

I'm not a fan of this helper: it has nothing to do with the networking,
actually it's a wrapper of get_mem_cgroup_from_kmem() replacing NULL
with root_mem_cgroup.

Overall the handling of root_mem_cgroup is very messy, I don't blame
this patch. But I wonder if it's better to simple move this code
to the call site without introducing a new function?

Alternatively, you can introduce something like:
struct mem_cgroup *mem_cgroup_or_root(struct mem_cgroup *memcg)
{
	return memcg ? memcg : root_mem_cgroup;
}

>  #else
>  static inline bool mem_cgroup_kmem_disabled(void)
>  {
> @@ -1763,9 +1786,13 @@ static inline void memcg_put_cache_ids(void)
>  
>  static inline struct mem_cgroup *mem_cgroup_from_obj(void *p)
>  {
> -       return NULL;
> +	return NULL;
>  }
>  
> +static inline struct mem_cgroup *get_net_memcg(void *p)
> +{
> +	return NULL;
> +}
>  #endif /* CONFIG_MEMCG_KMEM */
>  
>  #endif /* _LINUX_MEMCONTROL_H */
> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> index a5b5bb99c644..3093b4d5b2b9 100644
> --- a/net/core/net_namespace.c
> +++ b/net/core/net_namespace.c
> @@ -26,6 +26,7 @@
>  #include <net/net_namespace.h>
>  #include <net/netns/generic.h>
>  
> +#include <linux/sched/mm.h>
>  /*
>   *	Our network namespace constructor/destructor lists
>   */
> @@ -1147,7 +1148,13 @@ static int __register_pernet_operations(struct list_head *list,
>  		 * setup_net() and cleanup_net() are not possible.
>  		 */
>  		for_each_net(net) {
> +			struct mem_cgroup *old, *memcg;
> +
> +			memcg = get_net_memcg(net);
> +			old = set_active_memcg(memcg);
>  			error = ops_init(ops, net);
> +			set_active_memcg(old);
> +			mem_cgroup_put(memcg);
>  			if (error)
>  				goto out_undo;
>  			list_add_tail(&net->exit_list, &net_exit_list);
> -- 
> 2.31.1
>