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From: Roman Gushchin To: Yafang Shao Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin Lau , Song Liu , Yonghong Song , john fastabend , KP Singh , Stanislav Fomichev , Hao Luo , jolsa@kernel.org, Johannes Weiner , Michal Hocko , Shakeel Butt , Muchun Song , Andrew Morton , netdev , bpf , Linux MM Subject: Re: [PATCH bpf-next 13/15] mm, memcg: Add new helper get_obj_cgroup_from_cgroup Message-ID: References: <20220810151840.16394-1-laoar.shao@gmail.com> <20220810151840.16394-14-laoar.shao@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1660326029; a=rsa-sha256; cv=none; b=C4sRRoH44p+w5p2q8LViMZXPXO2xwBexCtmuINVXlbdHOTkvXe/NYk6Fq7imhsfNzB1XvR jFZVePf5p8VFw+8RzvgHIEdDXGIQwPK+iKhqY0y9ht4dsYfNHrT/yKYu3GmGDBBhYAaS+r nY7NhY8d/SEhyuJholkoyBrwgUI4Mj0= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=eo8EzCBX; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf22.hostedemail.com: domain of roman.gushchin@linux.dev designates 94.23.1.103 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=1660326029; 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-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=w6Ui6PFQyblCEyPZ57T42coFxoRognQXiZqVXoIWqFs=; b=0+2kg/zWBGTbJYjzUGI/jRedSTCBY20RhF45EA+sBbtydGlfHWqewxz2OZBnfxNXTTEBjH 00RsqbEzF2TwN8Nq8SymPgS+TuWwanGVtgBT0U7MPCmOa0ZHdb+vz2vEK6nSH0sXYVAFSR xq1bP5lJklvK2QO/noXrckp9yHDBNko= X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E5E35C007F Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=eo8EzCBX; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf22.hostedemail.com: domain of roman.gushchin@linux.dev designates 94.23.1.103 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev X-Rspam-User: X-Stat-Signature: hqdk8de51hu6nnwmh9jeuq8yf19c8s83 X-HE-Tag: 1660326028-769015 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: On Fri, Aug 12, 2022 at 08:35:19AM +0800, Yafang Shao wrote: > On Fri, Aug 12, 2022 at 12:16 AM Roman Gushchin > wrote: > > > > On Wed, Aug 10, 2022 at 03:18:38PM +0000, Yafang Shao wrote: > > > Introduce new helper get_obj_cgroup_from_cgroup() to get obj_cgroup from > > > a specific cgroup. > > > > > > Signed-off-by: Yafang Shao > > > --- > > > include/linux/memcontrol.h | 1 + > > > mm/memcontrol.c | 41 +++++++++++++++++++++++++++++++++++++++++ > > > 2 files changed, 42 insertions(+) > > > > > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > > > index 2f0a611..901a921 100644 > > > --- a/include/linux/memcontrol.h > > > +++ b/include/linux/memcontrol.h > > > @@ -1713,6 +1713,7 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg, > > > int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order); > > > void __memcg_kmem_uncharge_page(struct page *page, int order); > > > > > > +struct obj_cgroup *get_obj_cgroup_from_cgroup(struct cgroup *cgrp); > > > struct obj_cgroup *get_obj_cgroup_from_current(void); > > > struct obj_cgroup *get_obj_cgroup_from_page(struct page *page); > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 618c366..762cffa 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -2908,6 +2908,47 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg) > > > return objcg; > > > } > > > > > > +static struct obj_cgroup *get_obj_cgroup_from_memcg(struct mem_cgroup *memcg) > > > +{ > > > + struct obj_cgroup *objcg; > > > + > > > + if (memcg_kmem_bypass()) > > > + return NULL; > > > + > > > + rcu_read_lock(); > > > + objcg = __get_obj_cgroup_from_memcg(memcg); > > > + rcu_read_unlock(); > > > + return objcg; > > > > This code doesn't make sense to me. What does rcu read lock protect here? > > To protect rcu_dereference(memcg->objcg);. > Doesn't it need the read rcu lock ? No, it's not how rcu works. Please, take a look at the docs here: https://docs.kernel.org/RCU/whatisRCU.html#whatisrcu . In particular, it describes this specific case very well. In 2 words, you don't protect the rcu_dereference() call, you protect the pointer you get, cause it's valid only inside the rcu read section. After rcu_read_unlock() it might point at a random data, because the protected object can be already freed. Thanks!