From mboxrd@z Thu Jan 1 00:00:00 1970 From: Li Zefan Subject: Re: [RFC][PATCH 2/7] memcg: don't use mem_cgroup_get() when creating a kmemcg cache Date: Sun, 7 Apr 2013 11:32:48 +0800 Message-ID: <5160E8E0.2050602@huawei.com> References: <515BF233.6070308@huawei.com> <515BF275.5080408@huawei.com> <20130403153133.GM16471@dhcp22.suse.cz> <515EA73C.8050602@parallels.com> <20130405134557.GG31132@dhcp22.suse.cz> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130405134557.GG31132-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Michal Hocko Cc: Glauber Costa , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, LKML , Cgroups , Tejun Heo , KAMEZAWA Hiroyuki , Johannes Weiner >>> You are putting references but I do not see any single css_{try}get >>> here. /me puzzled. >>> >> >> There are two things being done in this code: >> First, we acquired a css_ref to make sure that the underlying cgroup >> would not go away. That is a short lived reference, and it is put as >> soon as the cache is created. >> At this point, we acquire a long-lived per-cache memcg reference count >> to guarantee that the memcg will still be alive. >> >> so it is: >> >> enqueue: css_get >> create : memcg_get, css_put >> destroy: css_put >> >> If I understand Li's patch correctly, he is not touching the first >> css_get, only turning that into the long lived reference (which was not >> possible before, since that would prevent rmdir). >> >> Then he only needs to get rid of the memcg_get, change the memcg_put to >> css_put, and get rid of the now extra css_put. >> >> He is issuing extra css_puts in memcg_create_kmem_cache, but only in >> failure paths. So the code reads as: >> * css_get on enqueue (already done, so not shown in patch) >> * if it fails, css_put >> * if it succeeds, don't do anything. This is already the long-lived >> reference count. put it at release time. > > OK, this makes more sense now. It is __memcg_create_cache_enqueue which > takes the reference and it is not put after this because it replaced > mem_cgroup reference counting. > Li, please put something along these lines into the changelog. This is > really tricky and easy to get misunderstand. > Yeah, I think I'll just steal Glauber's explanation as the changelog. > You can put my Acked-by then. > Thanks! >> The code looks correct, and of course, extremely simpler due to the >> use of a single reference. >> >> Li, am I right in my understanding that this is your intention? >>