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 6DFDDE6B271 for ; Tue, 23 Dec 2025 04:42:19 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8E87E6B008C; Mon, 22 Dec 2025 23:42:16 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 8B8346B0092; Mon, 22 Dec 2025 23:42:16 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 78B5D6B0093; Mon, 22 Dec 2025 23:42:16 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 603626B008C for ; Mon, 22 Dec 2025 23:42:16 -0500 (EST) Received: from smtpin13.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 0421C1404A6 for ; Tue, 23 Dec 2025 04:42:16 +0000 (UTC) X-FDA: 84249489072.13.D0DB406 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) by imf24.hostedemail.com (Postfix) with ESMTP id 563F8180005 for ; Tue, 23 Dec 2025 04:42:14 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=b0M+Jlw5; spf=pass (imf24.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.218.175.179 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1766464934; 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=TqbZvMEVprcnkrPOzqdX00WgU/AKJ/s7RLVFgkDEM6v2NtNsPUo1Ct0ZDgJbAnr/AX/ojx q5DwuOllV/LRx7XYKxrBVlQSh7/NWW799q7Gae5QRco54bT4cLQ566mDwBP6172jvllRle NtqK86WVC0ioqucWtDFEt4MAi2kh49c= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1766464934; a=rsa-sha256; cv=none; b=7ywDEhdt1fezNLkpDfe4PFQlKzeuoTfaxfFCEEfNdVficYV7rn/29Uuh0X/7JORumOS7Fq Trg0sBxQJ01x0fmc4qNbz8/lVGTTdpW9Gtm1jV/1TJMx3CfntY0FUNOCXe7kf745lYvcoz GGzZ/sUf+ozirihkZSNFnNygf/prJkc= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=b0M+Jlw5; spf=pass (imf24.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.218.175.179 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev 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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: 86yrgdgh33nnp19kgs1m13mx4e1bjska X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 563F8180005 X-Rspam-User: X-HE-Tag: 1766464934-302877 X-HE-Meta: U2FsdGVkX1+YtQ8mw2luXpdWWunYdq3vlwNqnlrGPAX+O6aLlWPjD+S5uLfLLPT9rP7Yx5G2QV9U0q6egD/CiwC1/MTQZ236kglceb3k8r8WvRrtsjD7jOQrn1oZy1tj+50ZBkgxO2ILyPgOlQrsGgvsGa+W4VCDTuRi/LYXAnnCKeTWitmSKiL8u2Ccmrb/a/qw3TWZd5J8vU0OCEYffpFW+cCJOjGqUCBvzAz8DBBgxE2iLQO4Gq7QvrEQ8JUS6/gnVxfoJtNvFYhkQWuz0RtMVm3axHRe3ePKyaY9D+twQ5h+J5AAMOAmm9TuW56cQb9dLEBiUyZrKMCv0WpDXHPMC9u3klszL9FJWP7VCLGekOVHkgbC4bYLSKnTP78I21qULoe9mEC2Np3uGFKlE3MwAAZ1lN6PvJu7URYifWHUkDqv2P21YmOG6+N2y05Iz2qIW8sq7T/VD0izhehQaoumbzT2fD5GqGAEDaq+WQtJ7BsAmM7JCK/R2f6+eVH6kjd337GROD75aWop8CCDQlpK/0bchNe5I+bNcgxyJ/K+wac79YKJdD4tpADqaIvjhDydfkWOhGYoXdKup1KuDmsmXzgwA2SnkXexp53jKLG53jxFLH1HgoHQEayjWVduCW4KnwfUn7nYlyf3FzgYRv8qky5OVV1fqoGUmws1cntt3NY7svrYdfR/KrsEYeGp7MJU+ZsN7+EvFjTLP2PrA1lIf3Z34jzms9Ug5ohwBJu3PcraXK02S/VVx5sRmWKzDWQHT8oGPwFt7d+eK8Q372Plf51E+2UplLr0t+AwnBf+ddx2R5rYSdHxou5duirTiKmtPngRZLTHytuQGz1Wm28xPhubDkz6mBc5xzFLA6VuOyjDDcoj9z82mDysfNtacjexEOzRwZvjbtWN3jMNfhb6gler6HAyZFWwCNvnYrCNqbQIPDQXRX9z4UQ60EGEFcjQyJ6TSmg01n6VLqZ OXAy7C0X KzpiAhkWEvfmuY2o3dPMNHhMHihLkCwHZlPbJ6jeY+ivSYMGTcYs3HLVrYAISKchzwKOWOml4rcqRAC+eToixT6FrpI/KzqpmLcboTODAw44IgKBoSYI+vErVntk+aBZ+sNcppG3YLYy7TjwzgFn1cwKYVKUJGn0DRHC3ayJrJlgUhCSm1qvrOfcDV3z+AJJTK0jlkbL/FJZGtZl09XZassnNum3gfxeXdXDHRbj958gspqzvUermv+Ib+g== 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