On Fri, Aug 12, 2022 at 12:48 AM Roman Gushchin
<roman.gushchin@xxxxxxxxx> wrote:
>
> On Wed, Aug 10, 2022 at 03:18:30PM +0000, Yafang Shao wrote:
> > The memcg may be the root_mem_cgroup, in which case we shouldn't put it.
> > So a new helper bpf_map_put_memcg() is introduced to pair with
> > bpf_map_get_memcg().
> >
> > Fixes: 4201d9ab3e42 ("bpf: reparent bpf maps on memcg offlining")
> > Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> > kernel/bpf/syscall.c | 14 +++++++++++---
> > 1 file changed, 11 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 83c7136..51ab8b1 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -441,6 +441,14 @@ static struct mem_cgroup *bpf_map_get_memcg(const struct bpf_map *map)
> > return root_mem_cgroup;
> > }
> >
> > +static void bpf_map_put_memcg(struct mem_cgroup *memcg)
> > +{
> > + if (mem_cgroup_is_root(memcg))
> > + return;
> > +
> > + mem_cgroup_put(memcg);
> > +}
>
> +1 to what Shakeel said. mem_cgroup_put(root_mem_cgroup) is totally valid.
> So this change does absolutely nothing.
>
Do you mean that we can mem_cgroup_put(root_mem_cgroup) without
mem_cgroup_get(root_mem_cgroup) ?
Am I missing something ?
> The fixes tag assumes there is a bug in the existing code. If so, please,
> describe the problem and how to reproduce it.
>
It is found by code review. The root_mem_cgroup's css will break. But
I don't know what it may cause to the user.
If you think the fixes tag is unproper, I will remove it.
> Also, if it's not related to the rest of the patchset, please, send it
> separately.
>
I want to introduce a bpf_map_put_memcg() helper to pair with
bpf_map_get_memcg().
This new helper will be used by other patches.
--
Regards
Yafang
