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[109.80.100.85]) by smtp.gmail.com with ESMTPSA id fx18-v6sm2834659ejb.31.2018.10.16.05.49.46 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 16 Oct 2018 05:49:46 -0700 (PDT) Date: Tue, 16 Oct 2018 14:49:39 +0200 From: Andrea Parri To: Kirill Tkhai Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm: Convert mem_cgroup_id::ref to refcount_t type Message-ID: <20181016124939.GA13278@andrea> References: <153910718919.7006.13400779039257185427.stgit@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <153910718919.7006.13400779039257185427.stgit@localhost.localdomain> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Kirill, On Tue, Oct 09, 2018 at 08:46:56PM +0300, Kirill Tkhai wrote: > This will allow to use generic refcount_t interfaces > to check counters overflow instead of currently existing > VM_BUG_ON(). The only difference after the patch is > VM_BUG_ON() may cause BUG(), while refcount_t fires > with WARN(). refcount_{sub_and_test,inc_not_zero}() are documented to provide "slightly" more relaxed ordering than their atomic_* counterpart, c.f., Documentation/core-api/refcount-vs-atomic.rst lib/refcount.c (inline comments) IIUC, this semantic change won't cause problems here (but please double-check? ;D ). Andrea >But this seems not to be significant here, > since such the problems are usually caught by syzbot > with panic-on-warn enabled. > > Signed-off-by: Kirill Tkhai > --- > include/linux/memcontrol.h | 2 +- > mm/memcontrol.c | 10 ++++------ > 2 files changed, 5 insertions(+), 7 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 4399cc3f00e4..7ab2120155a4 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -78,7 +78,7 @@ struct mem_cgroup_reclaim_cookie { > > struct mem_cgroup_id { > int id; > - atomic_t ref; > + refcount_t ref; > }; > > /* > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 7bebe2ddec05..aa728d5b3d72 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4299,14 +4299,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg) > > static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n) > { > - VM_BUG_ON(atomic_read(&memcg->id.ref) <= 0); > - atomic_add(n, &memcg->id.ref); > + refcount_add(n, &memcg->id.ref); > } > > static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n) > { > - VM_BUG_ON(atomic_read(&memcg->id.ref) < n); > - if (atomic_sub_and_test(n, &memcg->id.ref)) { > + if (refcount_sub_and_test(n, &memcg->id.ref)) { > mem_cgroup_id_remove(memcg); > > /* Memcg ID pins CSS */ > @@ -4523,7 +4521,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css) > } > > /* Online state pins memcg ID, memcg ID pins CSS */ > - atomic_set(&memcg->id.ref, 1); > + refcount_set(&memcg->id.ref, 1); > css_get(css); > return 0; > } > @@ -6357,7 +6355,7 @@ subsys_initcall(mem_cgroup_init); > #ifdef CONFIG_MEMCG_SWAP > static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg) > { > - while (!atomic_inc_not_zero(&memcg->id.ref)) { > + while (!refcount_inc_not_zero(&memcg->id.ref)) { > /* > * The root cgroup cannot be destroyed, so it's refcount must > * always be >= 1. >