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Fri, 29 Nov 2019 02:28:11 +0000 From: Roman Gushchin To: Michal Hocko CC: "linux-mm@kvack.org" , Andrew Morton , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , Christian Borntraeger , "stable@vger.kernel.org" Subject: Re: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction Thread-Topic: [PATCH] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction Thread-Index: AQHVo8HY/WnVvAZSgEqkb7BHNOXmHaedMHAAgACaToCAASswgIAAUmsAgAEQogCAARjFAA== Date: Fri, 29 Nov 2019 02:28:11 +0000 Message-ID: <20191129022806.GB68299@localhost.localdomain> References: <20191125185453.278468-1-guro@fb.com> <20191126092918.GB20912@dhcp22.suse.cz> <20191126184135.GA66034@localhost.localdomain> <20191127123225.GR20912@dhcp22.suse.cz> <20191127172724.GA67742@localhost.localdomain> <20191128094312.GF26807@dhcp22.suse.cz> In-Reply-To: <20191128094312.GF26807@dhcp22.suse.cz> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-clientproxiedby: MWHPR19CA0089.namprd19.prod.outlook.com (2603:10b6:320:1f::27) To BYAPR15MB2631.namprd15.prod.outlook.com (2603:10b6:a03:150::19) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c090:180::32b2] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 6fc1e111-d149-4bc2-2248-08d77473c747 x-ms-traffictypediagnostic: BYAPR15MB2533: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:345; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 6fc1e111-d149-4bc2-2248-08d77473c747 X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Nov 2019 02:28:11.2132 (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: M/5hMK9D6sp4IunSy7BTSl32m+rB5GR2SvgFHj36Zxh/KYZt5YJCoG8JslXGSuO2 X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2533 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,18.0.572 definitions=2019-11-28_08:2019-11-28,2019-11-28 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 priorityscore=1501 spamscore=0 clxscore=1015 lowpriorityscore=0 malwarescore=0 mlxscore=0 impostorscore=0 bulkscore=0 phishscore=0 suspectscore=0 adultscore=0 mlxlogscore=616 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-1911290019 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 Thu, Nov 28, 2019 at 10:43:12AM +0100, Michal Hocko wrote: > On Wed 27-11-19 17:27:29, Roman Gushchin wrote: > > On Wed, Nov 27, 2019 at 01:32:25PM +0100, Michal Hocko wrote: > > > On Tue 26-11-19 18:41:41, Roman Gushchin wrote: > > > > On Tue, Nov 26, 2019 at 10:29:18AM +0100, Michal Hocko wrote: > > > > > On Mon 25-11-19 10:54:53, Roman Gushchin wrote: > > > [...] > > > > > > So in a rare case when not all children kmem_caches are destroy= ed > > > > > > at the moment when the root kmem_cache is about to be gone, we = need > > > > > > to wait another rcu grace period before destroying the root > > > > > > kmem_cache. > > > > >=20 > > > > > Could you explain how rare this really is please? > > > >=20 > > > > It seems that we don't destroy root kmem_caches with enabled memcg > > > > accounting that often, but maybe I'm biased here. > > >=20 > > > So this happens each time a root kmem_cache is destroyed? Which would > > > imply that only dynamically created ones? > >=20 > > Yes, only dynamically created and only in those cases when destruction > > of the root cache happens immediately after the deactivation of the > > non-root cache. > > Tbh I can't imagine any other case except rmmod after > > removing the cgroup. >=20 > Thanks for the confirmation! Could you please make this explicit in the > changelog please? Maybe it is obvious to you but it took me quite some > time to grasp what the hell is going on here. Both memcg and kmem_cache > destruction are quite complex and convoluted. Sure, will send v2 shorty. Thanks!