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Mon, 5 Aug 2019 19:50:52 +0000 From: Roman Gushchin To: Michal Hocko CC: Andrew Morton , "linux-mm@kvack.org" , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , Hillf Danton Subject: Re: [PATCH v2] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Topic: [PATCH v2] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Index: AQHVSWe95NlTDGyLyE2eOxgDGjbby6bsakaAgACREIA= Date: Mon, 5 Aug 2019 19:50:52 +0000 Message-ID: <20190805195047.GA16917@tower.DHCP.thefacebook.com> References: <20190802192241.3253165-1-guro@fb.com> <20190805111135.GE7597@dhcp22.suse.cz> In-Reply-To: <20190805111135.GE7597@dhcp22.suse.cz> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-clientproxiedby: MWHPR21CA0054.namprd21.prod.outlook.com (2603:10b6:300:db::16) To DM6PR15MB2635.namprd15.prod.outlook.com (2603:10b6:5:1a6::24) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c090:200::e44] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: caf2e730-89cb-4a00-039a-08d719de3888 x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:DM6PR15MB2603; 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charset="us-ascii" Content-ID: <509D1DD3ADE9FD4788650A59BC9C7997@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: caf2e730-89cb-4a00-039a-08d719de3888 X-MS-Exchange-CrossTenant-originalarrivaltime: 05 Aug 2019 19:50:52.2813 (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: guro@fb.com X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR15MB2603 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-05_10:,, signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908050198 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 Mon, Aug 05, 2019 at 01:11:35PM +0200, Michal Hocko wrote: > On Fri 02-08-19 12:22:41, Roman Gushchin wrote: > > Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_charge= ") > > introduced css_tryget()/css_put() calls in drain_all_stock(), > > which are supposed to protect the target memory cgroup from being > > released during the mem_cgroup_is_descendant() call. > >=20 > > However, it's not completely safe. In theory, memcg can go away > > between reading stock->cached pointer and calling css_tryget(). > >=20 > > This can happen if drain_all_stock() races with drain_local_stock() > > performed on the remote cpu as a result of a work, scheduled > > by the previous invocation of drain_all_stock(). >=20 > Maybe I am still missing something but I do not see how 72f0184c8a00 > changed the existing race. get_online_cpus doesn't prevent the same race > right? If this is the case then it would be great to clarify that. I > know that you are mostly after clarifying that css_tryget is > insufficient but the above sounds like 72f0184c8a00 has introduced a > regression. Yeah, I'm not blaming 72f0184c8a00 for the race, which as I said, is barely reproducible at all. There is no "Fixes" tag, and I don't think we need to backport it to stable. Let's think about this patch as a refactoring patch, which makes the code cleaner. >=20 > > The race is a bit theoretical and there are few chances to trigger > > it, but the current code looks a bit confusing, so it makes sense > > to fix it anyway. The code looks like as if css_tryget() and > > css_put() are used to protect stocks drainage. It's not necessary > > because stocked pages are holding references to the cached cgroup. > > And it obviously won't work for works, scheduled on other cpus. > >=20 > > So, let's read the stock->cached pointer and evaluate the memory > > cgroup inside a rcu read section, and get rid of > > css_tryget()/css_put() calls. > >=20 > > v2: added some explanations to the commit message, no code changes > >=20 > > Signed-off-by: Roman Gushchin > > Cc: Michal Hocko > > Cc: Hillf Danton >=20 > Other than that. > Acked-by: Michal Hocko Thanks!