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

[Roman Gushchin]

On Tue, Nov 17, 2020 at 04:46:34PM -0800, Roman Gushchin wrote:
> 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?

Actually, this is problematic if a program updates several maps, because
in theory they can belong to different cgroups.
So it seems that the first option is the way to go. Do you agree?