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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> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: q8god1bssjrsboqaywesz4yk9w3i86bs X-Rspam-User: X-Rspamd-Queue-Id: 5149110000C X-Rspamd-Server: rspam05 X-HE-Tag: 1755536528-463851 X-HE-Meta: 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 5UEXnuJM vizLoZ9kzt9vd5azg8i34/q56TNYBFfqoGsmnHfUPOapEpViLBk46gydfi8oyqZhDCyjieanjwObzMtUZ8ZQYTc06tKFeywco9vno8r9c3gJqvSOwEw1I9gnW7tQ2l/QJJatYxeDtBqL9pIqfa5ZLcRkEsmyzX9SqDJzSQvth0Lc2r8n46+TOFrMQDsdhuUxqcuJCF2p8y6rNh/M= 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 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 @@ __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() + */ +__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(); 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) -- 2.50.1