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Sat, 20 Jun 2020 19:59:08 +0000 Date: Sat, 20 Jun 2020 12:59:05 -0700 From: Roman Gushchin To: Waiman Long CC: Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Johannes Weiner , Shakeel Butt , , Subject: Re: [PATCH] mm, slab: Fix sign conversion problem in memcg_uncharge_slab() Message-ID: <20200620195905.GA286724@carbon.DHCP.thefacebook.com> References: <20200620184719.10994-1-longman@redhat.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200620184719.10994-1-longman@redhat.com> X-ClientProxiedBy: BYAPR21CA0026.namprd21.prod.outlook.com (2603:10b6:a03:114::36) To BYAPR15MB4136.namprd15.prod.outlook.com (2603:10b6:a03:96::24) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from carbon.DHCP.thefacebook.com (2620:10d:c090:400::5:4a6b) by BYAPR21CA0026.namprd21.prod.outlook.com (2603:10b6:a03:114::36) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3153.2 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: 8fae9e8f-457f-411f-0474-08d8155464b8 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Jun 2020 19:59:08.6123 (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: XAeZzY/IbCIhNB6pmQQY1s+PK6s1ayEXvCQVyDk94rUIl+uezANXpJqK3WYDA2dF X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB3158 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216,18.0.687 definitions=2020-06-20_13:2020-06-19,2020-06-20 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 bulkscore=0 impostorscore=0 malwarescore=0 phishscore=0 mlxlogscore=999 spamscore=0 clxscore=1015 mlxscore=0 lowpriorityscore=0 priorityscore=1501 cotscore=-2147483648 adultscore=0 suspectscore=1 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006200149 X-FB-Internal: deliver X-Rspamd-Queue-Id: 93513180AD815 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 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 Sat, Jun 20, 2020 at 02:47:19PM -0400, Waiman Long wrote: > It was found that running the LTP test on a PowerPC system could produce > erroneous values in /proc/meminfo, like: > > MemTotal: 531915072 kB > MemFree: 507962176 kB > MemAvailable: 1100020596352 kB > > Using bisection, the problem is tracked down to commit 9c315e4d7d8c > ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()"). > > In memcg_uncharge_slab() with a "int order" argument: > > unsigned int nr_pages = 1 << order; > : > mod_lruvec_state(lruvec, cache_vmstat_idx(s), -nr_pages); > > The mod_lruvec_state() function will eventually call the > __mod_zone_page_state() which accepts a long argument. Depending on > the compiler and how inlining is done, "-nr_pages" may be treated as > a negative number or a very large positive number. Apparently, it was > treated as a large positive number in that PowerPC system leading to > incorrect stat counts. This problem hasn't been seen in x86-64 yet, > perhaps the gcc compiler there has some slight difference in behavior. > > It is fixed by making nr_pages a signed value. For consistency, a > similar change is applied to memcg_charge_slab() as well. > > Fixes: 9c315e4d7d8c ("mm: memcg/slab: cache page number in memcg_(un)charge_slab()"). > Signed-off-by: Waiman Long Good catch! Interesting that I haven't seen it on x86-64, but it's reproducible on Power. Acked-by: Roman Gushchin Thanks! > --- > mm/slab.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/slab.h b/mm/slab.h > index 207c83ef6e06..74f7e09a7cfd 100644 > --- a/mm/slab.h > +++ b/mm/slab.h > @@ -348,7 +348,7 @@ static __always_inline int memcg_charge_slab(struct page *page, > gfp_t gfp, int order, > struct kmem_cache *s) > { > - unsigned int nr_pages = 1 << order; > + int nr_pages = 1 << order; > struct mem_cgroup *memcg; > struct lruvec *lruvec; > int ret; > @@ -388,7 +388,7 @@ static __always_inline int memcg_charge_slab(struct page *page, > static __always_inline void memcg_uncharge_slab(struct page *page, int order, > struct kmem_cache *s) > { > - unsigned int nr_pages = 1 << order; > + int nr_pages = 1 << order; > struct mem_cgroup *memcg; > struct lruvec *lruvec; > > -- > 2.18.1 >