From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 09A08E6B257 for ; Mon, 22 Dec 2025 22:18:16 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 733B26B008C; Mon, 22 Dec 2025 17:18:15 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6B3826B0092; Mon, 22 Dec 2025 17:18:15 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 539016B0093; Mon, 22 Dec 2025 17:18:15 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id 3EF246B008C for ; Mon, 22 Dec 2025 17:18:15 -0500 (EST) Received: from smtpin19.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 0BD20B8F5E for ; Mon, 22 Dec 2025 22:18:15 +0000 (UTC) X-FDA: 84248521350.19.C811979 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) by imf16.hostedemail.com (Postfix) with ESMTP id 5FEA6180005 for ; Mon, 22 Dec 2025 22:18:13 +0000 (UTC) Authentication-Results: imf16.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=w5aeOZBV; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf16.hostedemail.com: domain of roman.gushchin@linux.dev designates 95.215.58.177 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1766441893; a=rsa-sha256; cv=none; b=17w/M//PSyHBsjpXpDywJackV/npSRyjxVHCsp231fXrIHtz+uGXJ6Nf4wOU+aOW9Vz8Yx CYOqUvK0gSCjck6x4ahNPh3UPx6kYxsRoDp+4VG8VSZrvy2UVadUhoYqQq9urKy/gg6Sd5 44Yj3lwBY5bEzph51H5iRhZsf3N92KA= ARC-Authentication-Results: i=1; imf16.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=w5aeOZBV; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf16.hostedemail.com: domain of roman.gushchin@linux.dev designates 95.215.58.177 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1766441893; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=a1GnAwrKnomf1xfbS9Wxii7iuVLuWoO/88OS8fu1I/I=; b=Xk6fDxveob0+K1Fm/XQYsPEAStnFZ8BGu2ccSiwbLg4t19rXemUmfs7qAtuir8KxMQqWo9 6GMWIEuH3n2jhfKU8eCcaMRQhgVXcd99gUTyb+Sea5NZ/8X366UMPAH9SCzL+EKsEgJCGn jvz+8+9VfxmIoaxrq8RirtEVhSoRnF0= 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=1766441892; 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=w5aeOZBV1ksO7V2W3ZBnV/+2HfXWaG53vmwIToGN7i2BQGf0Y4j8OBWnHWjJZIH+fXyAqJ bJaC4a4vVVsU2oMBtNmh+nFVtflQanwLrilDAqZzM0TXY4ZazryF1NETo3Ue/XnRsHcbx9 yYylTY3f/2Lnc6UwDq/G+07Xf7Btjo4= 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 v3 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Date: Mon, 22 Dec 2025 14:17:51 -0800 Message-ID: <20251222221754.186191-4-roman.gushchin@linux.dev> In-Reply-To: <20251222221754.186191-1-roman.gushchin@linux.dev> References: <20251222221754.186191-1-roman.gushchin@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5FEA6180005 X-Stat-Signature: iwpasntpfcc5hm5inubx8k4fifhtou34 X-Rspam-User: X-HE-Tag: 1766441893-791988 X-HE-Meta: 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 5SMA+xs6 OlTbAKeRkLo3vm/wcMNnWHP6zjgSSfK5NlFOiUFrMrlqBG/8HsYu4cIRmd4CB98tV6jpwdYiXdL8lY7lyNgXBBscTH6fKQBWiaMRYdLXARhoHATj0Ep01rRhRk9XPfcFv0Gt2ehEH/lBmbYVKxCT2jlSOBhwAkMFiF9IZFPiEm1hOCtOu41Sd8ZIDKBq6pybhQSquqFQLdfDb2lOvPoQIZdwH5iKiXuF++dnoK36timLLbXQXZCaZcc/I/Q== 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 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 @@ __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() + * + * 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 *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_ACQUIRE | KF_RET_NULL | KF_RCU) BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE) -- 2.52.0