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Fri, 2 Aug 2019 16:55:25 +0000 From: Roman Gushchin To: Hillf Danton CC: Andrew Morton , "linux-mm@kvack.org" , Michal Hocko , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team , "Michal Hocko" Subject: Re: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Topic: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Index: AQHVSMHRid8xy6v2VESi4846kGYnRKboFKCA Date: Fri, 2 Aug 2019 16:55:24 +0000 Message-ID: <20190802165521.GA28431@tower.DHCP.thefacebook.com> References: <20190801233513.137917-1-guro@fb.com> In-Reply-To: <20190801233513.137917-1-guro@fb.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-clientproxiedby: MWHPR13CA0016.namprd13.prod.outlook.com (2603:10b6:300:16::26) To DM6PR15MB2635.namprd15.prod.outlook.com (2603:10b6:5:1a6::24) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c090:200::a1bd] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 76f52506-952d-4c0c-bfa5-08d7176a36ad x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:DM6PR15MB2873; 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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: 76f52506-952d-4c0c-bfa5-08d7176a36ad X-MS-Exchange-CrossTenant-originalarrivaltime: 02 Aug 2019 16:55:25.0841 (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: DM6PR15MB2873 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-02_07:,, 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-1908020174 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 Fri, Aug 02, 2019 at 11:33:33AM +0800, Hillf Danton wrote: >=20 > On Thu, 1 Aug 2019 16:35:13 -0700 Roman Gushchin wrote: > >=20 > > 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 > Good catch! > >=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 > You need to either adjust the boundry of the rcu-protected section, or > retain the call pairs, as the memcg cache is dereferenced again in > drain_stock(). Not really. drain_stock() is always accessing the local percpu stock, and stock->cached memcg pointer is protected by references of stocked pages. Pages are stocked and drained only locally, so they can't go away. So if (stock->nr_pages > 0), the memcg has at least stock->nr_pages referen= ces. Also, because stocks on other cpus are drained via scheduled work, neither rcu_read_lock(), not css_tryget()/css_put() protects it. That's exactly the reason why I think this code is worth changing: it looks confusing. It looks like css_tryget()/css_put() protect stock draining, however it's not true. Thanks! > >=20 > > Signed-off-by: Roman Gushchin > > Cc: Michal Hocko > > --- > > mm/memcontrol.c | 17 +++++++++-------- > > 1 file changed, 9 insertions(+), 8 deletions(-) > >=20 > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index 5c7b9facb0eb..d856b64426b7 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -2235,21 +2235,22 @@ static void drain_all_stock(struct mem_cgroup *= root_memcg) > > for_each_online_cpu(cpu) { > > struct memcg_stock_pcp *stock =3D &per_cpu(memcg_stock, cpu); > > struct mem_cgroup *memcg; > > + bool flush =3D false; > > =20 > > + rcu_read_lock(); > > memcg =3D stock->cached; > > - if (!memcg || !stock->nr_pages || !css_tryget(&memcg->css)) > > - continue; > > - if (!mem_cgroup_is_descendant(memcg, root_memcg)) { > > - css_put(&memcg->css); > > - continue; > > - } > > - if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { > > + if (memcg && stock->nr_pages && > > + mem_cgroup_is_descendant(memcg, root_memcg)) > > + flush =3D true; > > + rcu_read_unlock(); > > + > > + if (flush && > > + !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { > > if (cpu =3D=3D curcpu) > > drain_local_stock(&stock->work); > > else > > schedule_work_on(cpu, &stock->work); > > } > > - css_put(&memcg->css); > > } > > put_cpu(); > > mutex_unlock(&percpu_charge_mutex); > > --=20 > > 2.21.0 > >=20 >=20 >=20