From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f199.google.com (mail-lj1-f199.google.com [209.85.208.199]) by kanga.kvack.org (Postfix) with ESMTP id 303A56B026C for ; Tue, 16 Oct 2018 09:19:51 -0400 (EDT) Received: by mail-lj1-f199.google.com with SMTP id a28-v6so6363774ljd.6 for ; Tue, 16 Oct 2018 06:19:51 -0700 (PDT) Received: from relay.sw.ru (relay.sw.ru. [185.231.240.75]) by mx.google.com with ESMTPS id 9-v6si13690686lja.3.2018.10.16.06.19.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Oct 2018 06:19:49 -0700 (PDT) Subject: Re: [PATCH] mm: Convert mem_cgroup_id::ref to refcount_t type References: <153910718919.7006.13400779039257185427.stgit@localhost.localdomain> <20181016124939.GA13278@andrea> From: Kirill Tkhai Message-ID: Date: Tue, 16 Oct 2018 16:19:40 +0300 MIME-Version: 1.0 In-Reply-To: <20181016124939.GA13278@andrea> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Parri Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Hi, Andrea, On 16.10.2018 15:49, Andrea Parri wrote: > 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 ). I just don't see a place, where we may think about using a modification of struct mem_cgroup::id::ref as a memory barrier to order something, and all this looks safe for me. Kirill >> 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. >>