linux-kernel - Re: [PATCH bpf-next v2] bpf: reparent bpf maps on memcg offlining

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Date:   Tue, 12 Jul 2022 15:11:02 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2] bpf: reparent bpf maps on memcg offlining

On Tue, Jul 12, 2022 at 2:49 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Jul 11, 2022 at 9:28 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> >
> > The memory consumed by a mpf map is always accounted to the memory
> > cgroup of the process which created the map. The map can outlive
> > the memory cgroup if it's used by processes in other cgroups or
> > is pinned on bpffs. In this case the map pins the original cgroup
> > in the dying state.
> >
> > For other types of objects (slab objects, non-slab kernel allocations,
> > percpu objects and recently LRU pages) there is a reparenting process
> > implemented: on cgroup offlining charged objects are getting
> > reassigned to the parent cgroup. Because all charges and statistics
> > are fully recursive it's a fairly cheap operation.
> >
> > For efficiency and consistency with other types of objects, let's do
> > the same for bpf maps. Fortunately thanks to the objcg API, the
> > required changes are minimal.
> >
> > Please, note that individual allocations (slabs, percpu and large
> > kmallocs) already have the reparenting mechanism. This commit adds
> > it to the saved map->memcg pointer by replacing it to map->objcg.
> > Because dying cgroups are not visible for a user and all charges are
> > recursive, this commit doesn't bring any behavior changes for a user.
> >
> > v2:
> >   added a missing const qualifier
> >
> > Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> > ---
> >  include/linux/bpf.h  |  2 +-
> >  kernel/bpf/syscall.c | 35 +++++++++++++++++++++++++++--------
> >  2 files changed, 28 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 2b21f2a3452f..85a4db3e0536 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -221,7 +221,7 @@ struct bpf_map {
> >         u32 btf_vmlinux_value_type_id;
> >         struct btf *btf;
> >  #ifdef CONFIG_MEMCG_KMEM
> > -       struct mem_cgroup *memcg;
> > +       struct obj_cgroup *objcg;
> >  #endif
> >         char name[BPF_OBJ_NAME_LEN];
> >         struct bpf_map_off_arr *off_arr;
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index ab688d85b2c6..ef60dbc21b17 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -419,35 +419,52 @@ void bpf_map_free_id(struct bpf_map *map, bool do_idr_lock)
> >  #ifdef CONFIG_MEMCG_KMEM
> >  static void bpf_map_save_memcg(struct bpf_map *map)
> >  {
> > -       map->memcg = get_mem_cgroup_from_mm(current->mm);
> > +       /* Currently if a map is created by a process belonging to the root
> > +        * memory cgroup, get_obj_cgroup_from_current() will return NULL.
> > +        * So we have to check map->objcg for being NULL each time it's
> > +        * being used.
> > +        */
> > +       map->objcg = get_obj_cgroup_from_current();
> >  }
> >
> >  static void bpf_map_release_memcg(struct bpf_map *map)
> >  {
> > -       mem_cgroup_put(map->memcg);
> > +       if (map->objcg)
> > +               obj_cgroup_put(map->objcg);
> > +}
> > +
> > +static struct mem_cgroup *bpf_map_get_memcg(const struct bpf_map *map) {
> > +       if (map->objcg)
> > +               return get_mem_cgroup_from_objcg(map->objcg);
> > +
> > +       return root_mem_cgroup;
> >  }
> >
> >  void *bpf_map_kmalloc_node(const struct bpf_map *map, size_t size, gfp_t flags,
> >                            int node)
> >  {
> > -       struct mem_cgroup *old_memcg;
> > +       struct mem_cgroup *memcg, *old_memcg;
> >         void *ptr;
> >
> > -       old_memcg = set_active_memcg(map->memcg);
> > +       memcg = bpf_map_get_memcg(map);
> > +       old_memcg = set_active_memcg(memcg);
> >         ptr = kmalloc_node(size, flags | __GFP_ACCOUNT, node);
> >         set_active_memcg(old_memcg);
> > +       mem_cgroup_put(memcg);
>
> Here we might css_put root_mem_cgroup.
> Should we css_get it when returning or
> it's marked as CSS_NO_REF ?
> But mem_cgroup_alloc() doesn't seem to be doing that marking.
> I'm lost at that code.

CSS_NO_REF is set for root_mem_cgroup in cgroup_init_subsys().