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Wed, 20 Aug 2025 02:25:39 -0700 (PDT) MIME-Version: 1.0 References: <20250818170136.209169-1-roman.gushchin@linux.dev> <20250818170136.209169-6-roman.gushchin@linux.dev> In-Reply-To: <20250818170136.209169-6-roman.gushchin@linux.dev> From: Kumar Kartikeya Dwivedi Date: Wed, 20 Aug 2025 11:25:02 +0200 X-Gm-Features: Ac12FXxpl9gi1KUaEwzxMMmXIWzdn3euZF0dat5BG2FZFZDhN6aQnB8C060Jz44 Message-ID: Subject: Re: [PATCH v1 05/14] mm: introduce bpf_get_root_mem_cgroup() bpf kfunc To: Roman Gushchin Cc: linux-mm@kvack.org, bpf@vger.kernel.org, Suren Baghdasaryan , Johannes Weiner , Michal Hocko , David Rientjes , Matt Bobrowski , Song Liu , Alexei Starovoitov , Andrew Morton , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: yih7tbnjsweyfqxidwcgc4j6y5nzp5hn X-Rspam-User: X-Rspamd-Queue-Id: 76B57140009 X-Rspamd-Server: rspam05 X-HE-Tag: 1755681941-691939 X-HE-Meta: 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 dqYWUYnT rkcqi1XJIVuVeBjyZfjfsyCgnh0U20Qlk8L5XUY/8yeM5Dtl8lS8Vobvvx7oMQEPmxIpy8D0Kfy7DzziXPjQ6iWXwVyRErzSHzObGkSjcWCglHnk05S2M4EVLkn31d89rMHUoH7Kjw5mvrpj7S9rVpEDhRp+ggW2qOQb+Bnlk2Q3IdSD59kkqDXIzmyxP627npPE6bIzu0EOm0A97oL4JyJfXcn13M/HarROSqyh/f5Q7a3BUP3HIHFHRpcheczLBJCdpZOloN+Y7q7EEbwDjI5jm+CMrZl0HCYQ9MA2SLOjDHkPmhJB+FsFgTuOYBzzLIaa7eHslOX7iQNSs6N+T/bXLOi4uo7QwpaSuL96gl+NR9fWPvVJk3o01N5PSiADJe446oCra4TjeNg4HQFLI6z25q9tnwUUa6rinV6pUl6mI5wo3B43zb8wutbafhvoOu4UQ3j/jUW1shkzAHOAnmr6S1B/E3SefBgzWZ8tPvq4fR9M= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 18 Aug 2025 at 19:02, 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 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 > --- > 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). > BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_ACQUIRE | KF_RET_NULL) > BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) > > -- > 2.50.1 >