On Tue, Jul 12, 2022 at 3:11 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Tue, Jul 12, 2022 at 2:49 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Mon, Jul 11, 2022 at 9:28 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> 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@xxxxxxxxx>
> > > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > > ---
> > > 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().
Ahh. I see that
css = ss->css_alloc(NULL); css->flags |= CSS_NO_REF; now.
Thanks.
