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

[Roman Gushchin <roman.gushchin@linux.dev>]

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.

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. 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.

Thanks!