From nobody Sat Oct 4 11:13:49 2025 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) (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 E3E193469F7 for ; Mon, 18 Aug 2025 17:02:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.188 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755536530; cv=none; b=HfemoKnROcEejwG9UKlESniEw1GkqGvkEiuFW3BFUHhCQaNeKOcEUA7q/pLognpQWOHZggyWHvYnw0Z0u4dRWBYpJsCkhwIaYyERjg04/l/3Rg9z5kigOBctqQ/gK94LbkHN3WuONyDMd75L+hrnAEZwu9ZJc5Mcv0O3jSzI3V4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755536530; c=relaxed/simple; bh=E2BhppgzFUlQ5lTqeZyZg+1Z/QXYkFBhZ9Y5K0aoKpQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Up49X+UOW8GC1daONpOu411relWfwMNamGxv1YLI4vrWZ4FG1zJb1A3yH/N0lFnMo2WSQVpwcYSDVCJPuaoTiljrGRIBApdTUipH1uuniTuoAppZpjj5boAEarhfalHOkt9naXoap7bwa4Sl5x+2WwDInL5PTuUt2FqAk6k6bRA= 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=UQf/ux/y; arc=none smtp.client-ip=95.215.58.188 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="UQf/ux/y" 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=1755536526; 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=a6r+LPu0pSFsz4Xm/eQ1z/XPznLAX+8OLgc0s/PVh14=; b=UQf/ux/y+q4m6txXDXG2RhzT0g0AxXDkgQwJ8Af7N6Bmd0yDMCdgna4YuRuOSEiaZnRX4x VjU4YYWKAFrW3w0CP6RuXC7F4PccfTvnLX6eStoaE9l31FapcAhFtZPb+oltmQ0qGtf75b k5ugS/MCzTGdcS9vUtDiOPKu0aiQYiw= From: Roman Gushchin To: linux-mm@kvack.org, bpf@vger.kernel.org Cc: Suren Baghdasaryan , Johannes Weiner , Michal Hocko , David Rientjes , Matt Bobrowski , Song Liu , Kumar Kartikeya Dwivedi , Alexei Starovoitov , Andrew Morton , linux-kernel@vger.kernel.org, Roman Gushchin Subject: [PATCH v1 05/14] mm: introduce bpf_get_root_mem_cgroup() bpf kfunc Date: Mon, 18 Aug 2025 10:01:27 -0700 Message-ID: <20250818170136.209169-6-roman.gushchin@linux.dev> In-Reply-To: <20250818170136.209169-1-roman.gushchin@linux.dev> References: <20250818170136.209169-1-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 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 @@ =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) +{ + /* 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(); =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) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) =20 --=20 2.50.1