linux-kernel - Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per

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Date:	Wed, 01 Jun 2011 15:07:40 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	linux-kernel@...r.kernel.org, kamezawa.hiroyu@...fujitsu.com,
	balbir@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_node
 to world

On 06/01/2011 02:39 PM, Michal Hocko wrote:
> On Wed 01-06-11 12:44:04, Igor Mammedov wrote:
>> Freshly allocated 'mem_cgroup_per_node' list entries must be
>> initialized before the rest of the kernel can see them. Otherwise
>> zero initialized list fields can lead to race condition at
>> mem_cgroup_force_empty_list:
>>    pc = list_entry(list->prev, struct page_cgroup, lru);
>> where 'pc' will be something like 0xfffffffc if list->prev is 0
>> and cause page fault later when 'pc' is dereferenced.
> Have you ever seen such a race? I do not see how this could happen.
> mem_cgroup_force_empty_list is called only from
> mem_cgroup_force_empty_write (aka echo whatever>  group/force_empty)
> or mem_cgroup_pre_destroy when the group is destroyed.
>
> The initialization code is, however, called before a group is
> given for use AFICS.
>
> I am not saying tha the change is bad, I like it, but I do not think it
> is a fix for potential race condition.
>

Yes I've seen it (RHBZ#700565). It causes random crashes
in virt env ocasionally. It's easier to reproduce if you overcommit
cpu.

>> Signed-off-by: Igor Mammedov<imammedo@...hat.com>
>> ---
>>   mm/memcontrol.c |    2 +-
>>   1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index bd9052a..ee7cb4c 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -4707,7 +4707,6 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
>>   	if (!pn)
>>   		return 1;
>>
>> -	mem->info.nodeinfo[node] = pn;
>>   	for (zone = 0; zone<  MAX_NR_ZONES; zone++) {
>>   		mz =&pn->zoneinfo[zone];
>>   		for_each_lru(l)
>> @@ -4716,6 +4715,7 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
>>   		mz->on_tree = false;
>>   		mz->mem = mem;
>>   	}
>> +	mem->info.nodeinfo[node] = pn;
>>   	return 0;
>>   }
>>
>> -- 
>> 1.7.1
>>
>> --
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