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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761607096; 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=0L0/Ocn9RqSsZN0Y29o3qrSHIg/Knm5QfgmpjPK++Dc=; b=hvHbWkeMCi+mGvHKY6QdSAqkwB8+dvLP330BknCQWQjexYhiDSjpkQh68nnGVIqt/WmOXA ym1pLJPCyIrwF+Fey/vwtrn/TcqXV/szfQ9TBW3DZxGU4AfLfCUEDoA0GDLWkq1yqy/SAz GwrAo1ztd805084vXph8luCWScY37+U= From: Roman Gushchin To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Alexei Starovoitov , Suren Baghdasaryan , Michal Hocko , Shakeel Butt , Johannes Weiner , Andrii Nakryiko , JP Kobryn , linux-mm@kvack.org, cgroups@vger.kernel.org, bpf@vger.kernel.org, Martin KaFai Lau , Song Liu , Kumar Kartikeya Dwivedi , Tejun Heo , Roman Gushchin Subject: [PATCH v2 09/23] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Date: Mon, 27 Oct 2025 16:17:12 -0700 Message-ID: <20251027231727.472628-10-roman.gushchin@linux.dev> In-Reply-To: <20251027231727.472628-1-roman.gushchin@linux.dev> References: <20251027231727.472628-1-roman.gushchin@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 63E6B1A000D X-Stat-Signature: u5ikudc34byo1frjew6oq9y4w4dzxbfe X-Rspam-User: X-HE-Tag: 1761607097-574 X-HE-Meta: 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 ckyGkxxz 74U8lA6kbHhyjj4m5m+anfeq0PQ/70Grfflf/c5NdHrmDtAvcF7be4UA5vfWUqMbhxV6QfhHNu+BrsTx/2CgZph4rfO4Ql2AWI0dvezuXmeD8AyoTvwZXtbQhgAIsL6ALsOxyFsthWF2wd9cDguwUG6pNtn7QqgWjXhNI67P1S4Fx4GTgw2i2okwEu4V9o7+SVrlZmiAytrNOc2o= 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 1e46097745cf..76c342318256 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 @@ -64,6 +78,7 @@ __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) -- 2.51.0