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Tue, 27 Oct 2020 19:05:54 +0000 Date: Tue, 27 Oct 2020 12:05:49 -0700 From: Roman Gushchin To: Muchun Song CC: , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg Message-ID: <20201027190549.GC827280@carbon.dhcp.thefacebook.com> References: <20201027080256.76497-1-songmuchun@bytedance.com> <20201027080256.76497-2-songmuchun@bytedance.com> <20201027174236.GB725724@carbon.dhcp.thefacebook.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201027174236.GB725724@carbon.dhcp.thefacebook.com> X-Originating-IP: [2620:10d:c090:400::4:e2f0] X-ClientProxiedBy: MWHPR18CA0064.namprd18.prod.outlook.com (2603:10b6:300:39::26) To BYAPR15MB4136.namprd15.prod.outlook.com (2603:10b6:a03:96::24) X-MS-Exchange-MessageSentRepresentingType: 1 Received: from carbon.dhcp.thefacebook.com (2620:10d:c090:400::4:e2f0) by MWHPR18CA0064.namprd18.prod.outlook.com (2603:10b6:300:39::26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3477.24 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: 7e24510e-8a4c-48db-8191-08d87aab5442 X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB4136.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Oct 2020 19:05:54.5795 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: daSaThPUabFNzzL1xXTzFVoIOxmhZRZacfeJPl/Qi4yshJ+ZOp19DVVTuEXNwe4h X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2278 X-OriginatorOrg: fb.com X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.312,18.0.737 definitions=2020-10-27_10:2020-10-26,2020-10-27 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxlogscore=999 lowpriorityscore=0 spamscore=0 malwarescore=0 adultscore=0 suspectscore=0 phishscore=0 clxscore=1015 impostorscore=0 priorityscore=1501 mlxscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2010270110 X-FB-Internal: deliver 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 Tue, Oct 27, 2020 at 10:42:36AM -0700, Roman Gushchin wrote: > On Tue, Oct 27, 2020 at 04:02:52PM +0800, Muchun Song wrote: > > Consider the following memcg hierarchy. > > > > root > > / \ > > A B > > > > If we get the objcg of memcg A failed, the get_obj_cgroup_from_current > > can return the wrong objcg for the root memcg. > > > > Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API") > > Signed-off-by: Muchun Song > > --- > > mm/memcontrol.c | 5 ++++- > > 1 file changed, 4 insertions(+), 1 deletion(-) > > Hi Muchun, > > thank you for the patchset! > > A generic note: it's usually not a good idea to group fixes with non-fixes > in one patchset if fixes are planned to be backported to stable. > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 1337775b04f3..fcbd79c5023e 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -2945,7 +2945,7 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p) > > > > __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void) > > { > > - struct obj_cgroup *objcg = NULL; > > + struct obj_cgroup *objcg; > > struct mem_cgroup *memcg; > > > > if (memcg_kmem_bypass()) > > @@ -2964,6 +2964,9 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void) > > } > > rcu_read_unlock(); > > > > + if (memcg == root_mem_cgroup) > > + return NULL; > > + > > return objcg; > > } > > I agree, it's a bug. Good catch, thanks! > > I would prefer a slightly different fix though. Because we're going to change how > the root memory cgroup is handled (an rfc version posted here: > https://lkml.org/lkml/2020/10/21/612), it's better to not use a comparison > with the root_mem_cgroup and handle the obj_cgroup_tryget() return code instead. > Something like this: > > @@ -2973,6 +2973,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void) > objcg = rcu_dereference(memcg->objcg); > if (objcg && obj_cgroup_tryget(objcg)) > break; > + objcg = NULL; > } > rcu_read_unlock(); > > Or a more explicit: > if (obj_cgroup_tryget(objcg)) { > ... > } else { > ... > } > > Also, please, add: > Cc: stable@vger.kernel.org After a second thought, it's also a highly theoretical race. In order to have obj_cgroup_tryget() failed we need to enter get_obj_cgroup_from_current() racing with cgroup offlining (which is already tricky) and then have memcg_reparent_objcgs() finish in between reading memcg->objcg and bumping the reference counter. So I'm not sure we need a stable@ backport, sorry for the confusion. Thanks!