From nobody Mon Feb 9 08:34:16 2026 Received: from out-172.mta0.migadu.com (out-172.mta0.migadu.com [91.218.175.172]) (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 8438C2EC56D for ; Sat, 20 Dec 2025 04:13:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.172 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766204002; cv=none; b=T/xYaUg72ew2bn+LeUDv4wBIInVrf31/dJUUVXzWFiZlHplWt6EZdhz7k9fIPHzI1GMPxABVWRJwL3V0k5Ms6Qw0587wUGVZ+jvpfNYGGtBL+K3a3+4Yxq+8IN2EcRKrE40BsWoIpXK8qWfQhO270gWVwQqLKn5aDAHHineg2Vs= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766204002; c=relaxed/simple; bh=TLFN6xvqCudM+BlZuTtt8seZTGFz5HGzdN9cbr5pse8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aCajzfD4fD6g5DyjM1jl2+Hp78zr3C/xlZYRqU71l6RQpn+j/Q5H/aFwlG+18+qqYWpr6Qmdsb58UEJc1JDJi8Mh3LsdtMYmEcMlKkvncFU8Lz4/XSars6xx4CqPGOTMJYtHtOp1Zd8WnOpGmwgYiHbot9/n8BMvYb8JtEmB4oM= 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=gKvEL66b; arc=none smtp.client-ip=91.218.175.172 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="gKvEL66b" 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=1766203997; 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=0lVuR6pchH6jgQ09wItDn6PsrwL4FoyScJ2gG8GX4fA=; b=gKvEL66bO+z7ttjD2KYiVENpRi3zKONK7yhd9JuzI2g+oZjmwihxs7aJ8zy+GjcaQsorjQ MX1CKTmzFk0hY+p8+U7H8JUqNp7fr2kYYOY7Zra6m6YZh5ImZjq8aYES1mnZX+YNu3CyGj l3bhS1a+4gZYizhA2A/fnDbB2iNuxEk= 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 v2 3/7] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Date: Fri, 19 Dec 2025 20:12:46 -0800 Message-ID: <20251220041250.372179-4-roman.gushchin@linux.dev> In-Reply-To: <20251220041250.372179-1-roman.gushchin@linux.dev> References: <20251220041250.372179-1-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 | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c index 03d435fc4f10..2d518ad2ad3f 100644 --- a/mm/bpf_memcontrol.c +++ b/mm/bpf_memcontrol.c @@ -10,6 +10,23 @@ =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() + */ +__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 +81,7 @@ __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_TRUSTED_ARGS | KF_ACQUIRE | KF_R= ET_NULL | KF_RCU) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_TRUSTED_ARGS | KF_RELEASE) =20 --=20 2.52.0