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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766109492; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ZxZtYJx1dwxpCMcKq8wqCyQtoClSuF/V4BGHii44wWo=; b=BEoP7ESdiETV53sORIkPZJVBHEIwFuS/bpau8YXo1EvZYX6rGF9DuhyiI6GxHGnBDZaeHw yRpXvo1Pl/R2nXI/bUT7XylqCrB6zVHds+oXksrWBJkx5djvWm59vH4fZFH7bBeGDAx7Tf IDxT5bqFE6e6lVkbruuiWLw359I9kU0= From: Roman Gushchin To: bpf@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: JP Kobryn , Alexei Starovoitov , Daniel Borkmann , Shakeel Butt , Michal Hocko , Johannes Weiner , Roman Gushchin Subject: [PATCH bpf-next v1 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Date: Thu, 18 Dec 2025 17:57:47 -0800 Message-ID: <20251219015750.23732-4-roman.gushchin@linux.dev> In-Reply-To: <20251219015750.23732-1-roman.gushchin@linux.dev> References: <20251219015750.23732-1-roman.gushchin@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam02 X-Stat-Signature: kqxnt6a716ougro1q15uyq51rixfkpeg X-Rspam-User: X-Rspamd-Queue-Id: D4F9A1C0012 X-HE-Tag: 1766109494-8263 X-HE-Meta: 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 68NyFoR5 Enb0wgjgDi9CQWHGxMPbo5fCJivs3BknEI52P7XQ59r5+lj0y+FNNi2XOZSa9e4RS8FNFTXMLQdPHCxkgPEEQHo/m/b7mpMCeFW4G79hUWY8/OjPYo5OZpZoXE9RhMz7PfvW7T9xFBA+rm/eIaEkBvowmTm5y6V3vHoO5+LsIg0QdDQvRS0oDhrMmLlF+xAMEn8ZMaJATxHKkdBZuGety5AaheGHZ46orvF56v0eLFo3sTc07msYvguTNHw== 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: 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 8aa842b56817..6d0d73bf0dd1 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 @@ -64,6 +78,7 @@ __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) BTF_ID_FLAGS(func, bpf_get_mem_cgroup, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL | KF_RCU) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_TRUSTED_ARGS | KF_RELEASE) -- 2.52.0