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dkim=pass header.d=gmail.com header.s=20210112 header.b=MEXEaE6E; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf13.hostedemail.com: domain of laoar.shao@gmail.com designates 209.85.217.46 as permitted sender) smtp.mailfrom=laoar.shao@gmail.com X-Stat-Signature: chpqc8zjeaayb6o4y7xp64pycukzkiuz X-Rspam-User: X-HE-Tag: 1660348652-162386 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 Sat, Aug 13, 2022 at 1:40 AM Roman Gushchin wrote: > > 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 I just copied and pasted rcu_dereference(memcg->objcg) there to make it clear. Actually it protects memcg->objcg, doesn't it ? > 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. > Are you sure? Can't the obj_cgroup_tryget(objcg) prevent it from being freed ? -- Regards Yafang