Re: [PATCH bpf-next v6 06/34] bpf: prepare for memcg-based memory accounting for bpf maps

[Roman Gushchin]

On Wed, Nov 18, 2020 at 01:06:17AM +0100, Daniel Borkmann wrote:
> On 11/17/20 4:40 AM, Roman Gushchin wrote:
> > In the absolute majority of cases if a process is making a kernel
> > allocation, it's memory cgroup is getting charged.
> > 
> > Bpf maps can be updated from an interrupt context and in such
> > case there is no process which can be charged. It makes the memory
> > accounting of bpf maps non-trivial.
> > 
> > Fortunately, after commit 4127c6504f25 ("mm: kmem: enable kernel
> > memcg accounting from interrupt contexts") and b87d8cefe43c
> > ("mm, memcg: rework remote charging API to support nesting")
> > it's finally possible.
> > 
> > To do it, a pointer to the memory cgroup of the process which created
> > the map is saved, and this cgroup is getting charged for all
> > allocations made from an interrupt context.
> > 
> > Allocations made from a process context will be accounted in a usual way.
> > 
> > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > Acked-by: Song Liu <songliubraving@xxxxxx>
> [...]
> > +#ifdef CONFIG_MEMCG_KMEM
> > +static __always_inline int __bpf_map_update_elem(struct bpf_map *map, void *key,
> > +						 void *value, u64 flags)
> > +{
> > +	struct mem_cgroup *old_memcg;
> > +	bool in_interrupt;
> > +	int ret;
> > +
> > +	/*
> > +	 * If update from an interrupt context results in a memory allocation,
> > +	 * the memory cgroup to charge can't be determined from the context
> > +	 * of the current task. Instead, we charge the memory cgroup, which
> > +	 * contained a process created the map.
> > +	 */
> > +	in_interrupt = in_interrupt();
> > +	if (in_interrupt)
> > +		old_memcg = set_active_memcg(map->memcg);
> > +
> > +	ret = map->ops->map_update_elem(map, key, value, flags);
> > +
> > +	if (in_interrupt)
> > +		set_active_memcg(old_memcg);
> > +
> > +	return ret;
> 
> Hmm, this approach here won't work, see also commit 09772d92cd5a ("bpf: avoid
> retpoline for lookup/update/delete calls on maps") which removes the indirect
> call, so the __bpf_map_update_elem() and therefore the set_active_memcg() is
> not invoked for the vast majority of cases.

I see. Well, the first option is to move these calls into map-specific update
functions, but the list is relatively long:
  nsim_map_update_elem()
  cgroup_storage_update_elem()
  htab_map_update_elem()
  htab_percpu_map_update_elem()
  dev_map_update_elem()
  dev_map_hash_update_elem()
  trie_update_elem()
  cpu_map_update_elem()
  bpf_pid_task_storage_update_elem()
  bpf_fd_inode_storage_update_elem()
  bpf_fd_sk_storage_update_elem()
  sock_map_update_elem()
  xsk_map_update_elem()

Alternatively, we can set the active memcg for the whole duration of bpf
execution. It's simpler, but will add some overhead. Maybe we can somehow
mark programs calling into update helpers and skip all others?

What do you think?

Thanks!