Re: [PATCH bpf-next v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

[Matt Bobrowski <mattbobrowski@google.com>]

On Tue, Dec 30, 2025 at 09:00:28PM +0000, Roman Gushchin wrote:
> Matt Bobrowski <mattbobrowski@google.com> writes:
> 
> > On Mon, Dec 22, 2025 at 08:41:53PM -0800, Roman Gushchin wrote:
> >> Introduce a BPF kfunc to get a trusted pointer to the root memory
> >> cgroup. It's very handy to traverse the full memcg tree, e.g.
> >> for handling a system-wide OOM.
> >> 
> >> It's possible to obtain this pointer by traversing the memcg tree
> >> up from any known memcg, but it's sub-optimal and makes BPF programs
> >> more complex and less efficient.
> >> 
> >> bpf_get_root_mem_cgroup() has a KF_ACQUIRE | KF_RET_NULL semantics,
> >> however in reality it's not necessary to bump the corresponding
> >> reference counter - root memory cgroup is immortal, reference counting
> >> is skipped, see css_get(). Once set, root_mem_cgroup is always a valid
> >> memcg pointer. It's safe to call bpf_put_mem_cgroup() for the pointer
> >> obtained with bpf_get_root_mem_cgroup(), it's effectively a no-op.
> >> 
> >> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> >> ---
> >>  mm/bpf_memcontrol.c | 20 ++++++++++++++++++++
> >>  1 file changed, 20 insertions(+)
> >> 
> >> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
> >> index 82eb95de77b7..187919eb2fe2 100644
> >> --- a/mm/bpf_memcontrol.c
> >> +++ b/mm/bpf_memcontrol.c
> >> @@ -10,6 +10,25 @@
> >>  
> >>  __bpf_kfunc_start_defs();
> >>  
> >> +/**
> >> + * bpf_get_root_mem_cgroup - Returns a pointer to the root memory cgroup
> >> + *
> >> + * The function has KF_ACQUIRE semantics, even though the root memory
> >> + * cgroup is never destroyed after being created and doesn't require
> >> + * reference counting. And it's perfectly safe to pass it to
> >> + * bpf_put_mem_cgroup()
> >> + *
> >> + * Return: A pointer to the root memory cgroup.
> >> + */
> >> +__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
> >> +{
> >> +	if (mem_cgroup_disabled())
> >> +		return NULL;
> >> +
> >> +	/* css_get() is not needed */
> >> +	return root_mem_cgroup;
> >> +}
> >> +
> >>  /**
> >>   * bpf_get_mem_cgroup - Get a reference to a memory cgroup
> >>   * @css: pointer to the css structure
> >> @@ -64,6 +83,7 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
> >>  __bpf_kfunc_end_defs();
> >>  
> >>  BTF_KFUNCS_START(bpf_memcontrol_kfuncs)
> >> +BTF_ID_FLAGS(func, bpf_get_root_mem_cgroup, KF_ACQUIRE | KF_RET_NULL)
> >
> > I feel as though relying on KF_ACQUIRE semantics here is somewhat
> > odd. Users of this BPF kfunc will now be forced to call
> > bpf_put_mem_cgroup() on the returned root_mem_cgroup, despite it being
> > completely unnecessary.
> 
> A agree that it's annoying, but I doubt this extra call makes any
> difference in the real world.

Sure, that certainly holds true.

> Also, the corresponding kernel code designed to hide the special
> handling of the root cgroup. css_get()/css_put() are simple no-ops for
> the root cgroup, but are totally valid.

Yes, I do see that.

> So in most places the root cgroup is handled as any other, which
> simplifies the code. I guess the same will be true for many bpf
> programs.

I see, however the same might not necessarily hold for all other
global pointers which end up being handed out by a BPF kfunc (not
necessarily bpf_get_root_mem_cgroup()). This is why I was wondering
whether there's some sense to introducing another KF flag (or
something similar) which allows returned values from BPF kfuncs to be
implicitly treated as trusted.