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

[Igor Mammedov]

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@xxxxxxxxxx>  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. 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?

--
Thanks,
 Igor
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