Re: [PATCH] memcg: do not expose uninitialized mem_cgroup_per_nodeto world

[Igor Mammedov]

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@xxxxxxxxxx>
> > ---
> >   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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