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charset="iso-8859-1" Content-ID: <976747A0B33EBD4A92BBDD26F233AC3A@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 5e663491-3b6f-49fb-d017-08d7685c1aa8 X-MS-Exchange-CrossTenant-originalarrivaltime: 13 Nov 2019 17:08:29.2613 (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: mIsgv7w1Jn1P+txumkm88km72Ko6QMN7C4ZJ0MpOwWBZZxA4o4uxpwCNPCkK6M+d X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN8PR15MB2852 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.95,18.0.572 definitions=2019-11-13_04:2019-11-13,2019-11-13 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 phishscore=0 adultscore=0 lowpriorityscore=0 mlxlogscore=999 bulkscore=0 clxscore=1015 priorityscore=1501 mlxscore=0 malwarescore=0 spamscore=0 suspectscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-1911130148 X-FB-Internal: deliver Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Wed, Nov 13, 2019 at 05:29:34PM +0100, Michal Koutn=FD wrote: > Hi. >=20 > On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin wr= ote: > > Let's fix it by switching from css_tryget_online() to css_tryget(). > Is this a safe thing to do? The stack captures a kmem charge path, with > css_tryget() it may happen it gets an offlined memcg and carry out > charge into it. What happens when e.g. memcg_deactivate_kmem_caches is > skipped as a consequence? The thing here is that css_tryget_online() cannot pin the online state, so even if returned true, the cgroup can be offline at the return from the function. So if we rely somewhere on it, it's already broken. Generally speaking, it's better to reduce it's usage to the bare minimum. >=20 > > The problem is caused by an exiting task which is associated with > > an offline memcg. We're iterating over and over in the > > do {} while (!css_tryget_online()) loop, but obviously the memcg won't > > become online and the exiting task won't be migrated to a live memcg. > As discussed in other replies, the task is not yet exiting. However, the > access to memcg isn't through `current` but `mm->owner`, i.e. another > task of a threadgroup may have got stuck in an offlined memcg (I don't > have a good explanation for that though). Yes, it's true, and I've no idea how the memcg can be offline in this case = too. We've seen it only several times in fb production, so it seems to be a real= ly rare case. Could be anything from a tiny race somewhere to cpu bugs. Thanks!