[PATCH] memcg: do not expose uninitialized mem_cgroup_per_node to world

[Michal Hocko]

On Wed 27-07-11 11:30:08, Igor Mammedov wrote:
> On 07/27/2011 09:58 AM, Michal Hocko wrote:
> >On Tue 26-07-11 14:17:54, Andrew Morton wrote:
> >>On Fri, 10 Jun 2011 18:57:40 +0200
> >>Igor Mammedov<imammedo at redhat.com>  wrote:
> >>
> >>>On 06/08/2011 11:09 PM, Andrew Morton wrote:
> >>>>The original patch:
> >>>>
> >>>>--- 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;
> >>>>   }
> >>>>
> >>>>looks like a really good idea.  But it needs a new changelog and I'd be
> >>>>a bit reluctant to merge it as it appears that the aptch removes our
> >>>>only known way of reproducing a bug.
> >>>>
> >>>>So for now I think I'll queue the patch up unchangelogged so the issue
> >>>>doesn't get forgotten about.
> >>>>
> >>>Problem was in rhel's xen hv.
> >>>It was missing fix for imul emulation.
> >>>Details here
> >>>http://lists.xensource.com/archives/html/xen-devel/2011-06/msg00801.html
> >>>Thanks to Tim Deegan and everyone who was involved in the discussion.
> >>
> >>I really don't want to trawl through a lengthy xen bug report
> >
> >The bug turned out to be Xen specific and this patch just hidden the bug
> >in Xen.
> 
> The problem was in incorrect imul instruction emulation in xen and as
> consequence incorrect attempt to initialize list at invalid memory location.
> 
> >
> >>and write your changelog for you.
> >>
> >>We still have no changelog for this patch.  Please send one.
> >
> >Appart from a better programming style is there any other reason for
> >taking it?  If applied it might hide potential bugs when somebody is
> >touching data too early.
> >
> 
> If it ever happens and somebody is touching data too early, it would be
> a bit easier to diagnose a problem when dereferencing NULL at
> mem->info.nodeinfo[node] than wondering at partly initialized
> mem_cgroup_per_zone. 

Bahh, I have mixed zero initialized with NULL that would blow up. You
are right of course, sorry for confusion.

> Aside from that it is purely cosmetic change.
> 
> Here is proposed change log:
> 
> Subject: Cleanup: memcg: Expose only initialized mem_cgroup_per_node to world
> 
> If somebody is touching data too early, it might be easier to diagnose
> a problem when dereferencing NULL at mem->info.nodeinfo[node] than
> trying to understand why mem_cgroup_per_zone is [un|partly]initialized.
> 
> 
> Michal will you agree with such commit message?

Acked-by: Michal Hocko <mhocko at suse.cz>

Thanks.
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic