From nobody Sat Feb 7 06:14:22 2026 Received: from out-181.mta0.migadu.com (out-181.mta0.migadu.com [91.218.175.181]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B05012E2299 for ; Tue, 23 Dec 2025 04:42:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.181 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766464936; cv=none; b=VdeU+o5ons6stxV1SZr34LWnDLbqwJkENYLspyR+DKsftiqsTilHQnDLZYz3N8FPpRI9sAcoYCPixlmjjVDaLeV30rkfXb1X/N5l+l1RxCkAhbIrJwbQQLOJH924tMb14F+ZiWszBUlTyRvT5zxo3a4rXPdVNxCkKieLMSyqdrg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766464936; c=relaxed/simple; bh=dh7KtVzts7zVSjOGS6x4foAi7gvq3KBUUwnBt8q1TUQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=XFGeCyG/BpMh8otiaEFAJqnPNLA5QL6bDGeBnrIwSgCLYDXbZ8a9fnmpjYOlvjig+nvRrn8XFuBpKoBqasM8Htk+vR+K6OejTJHfQ7UQi29CsZx0peCxlnKwFY08d9Bz+KJ/5UJu9VyA4IIUo0Nj4atCjq2LhD47fL2amooCOjc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=b0M+Jlw5; arc=none smtp.client-ip=91.218.175.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="b0M+Jlw5" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766464932; 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=a1GnAwrKnomf1xfbS9Wxii7iuVLuWoO/88OS8fu1I/I=; b=b0M+Jlw5AXuY6utYcm/etnGH2dLvH3B+1ZEptzn/VL0ztH69oHIeWmds7BWtx4A/Bu5f0K cQp/58uyq4QYzVQf4OM2kcT0lIXVvnMPFHKF+YfHp/V5Qb1zgRiXUNLGMmWdAjx2e4/mvw 48P20+NEVs4ixFli4oComEhp86K4ryk= 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 v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Date: Mon, 22 Dec 2025 20:41:53 -0800 Message-ID: <20251223044156.208250-4-roman.gushchin@linux.dev> In-Reply-To: <20251223044156.208250-1-roman.gushchin@linux.dev> References: <20251223044156.208250-1-roman.gushchin@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" 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 --- 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 @@ =20 __bpf_kfunc_start_defs(); =20 +/** + * 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 *me= mcg) __bpf_kfunc_end_defs(); =20 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_ACQUIRE | KF_RET_NULL | KF_RCU) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) =20 --=20 2.52.0