Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> writes:
> On Mon, 18 Aug 2025 at 19:02, Roman Gushchin <roman.gushchin@xxxxxxxxx> 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 necessarily 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@xxxxxxxxx>
>> ---
>> mm/bpf_memcontrol.c | 15 +++++++++++++++
>> 1 file changed, 15 insertions(+)
>>
>> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
>> index 66f2a359af7e..a8faa561bcba 100644
>> --- a/mm/bpf_memcontrol.c
>> +++ b/mm/bpf_memcontrol.c
>> @@ -10,6 +10,20 @@
>>
>> __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()
>> + */
>> +__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
>> +{
>> + /* 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
>> @@ -122,6 +136,7 @@ __bpf_kfunc void bpf_mem_cgroup_flush_stats(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)
>
> Same suggestion here (re: trusted args).
It's not really taking any arguments, so I don't think it's applicable:
struct mem_cgroup *bpf_get_root_mem_cgroup(void)
