Re: [Bugfix] sched: fix possible invalid memory access caused by CPU hot-addition

From: David Rientjes
Date: Tue Apr 22 2014 - 21:59:24 EST


On Tue, 22 Apr 2014, Peter Zijlstra wrote:

> On Tue, Apr 22, 2014 at 01:01:51PM -0700, Andrew Morton wrote:
> > On Tue, 22 Apr 2014 10:15:15 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > > On Tue, Apr 22, 2014 at 01:27:15PM +0800, Jiang Liu wrote:
> > > > When calling kzalloc_node(size, flags, node), we should first check
> > > > whether node is onlined, otherwise it may cause invalid memory access
> > > > as below.
> > >
> > > But this is only for memory less node crap, right?
> >
> > um, why are memoryless nodes crap?
>
> Why wouldn't they be? Having CPUs with no local memory seems decidedly
> suboptimal.

The quick fix for memoryless node issues is usually just do cpu_to_mem()
rather than cpu_to_node() in the caller. This assumes that the arch is
setup correctly to handle memoryless nodes with
CONFIG_HAVE_MEMORYLESS_NODES (and we've had problems recently with
memoryless nodes not being configured correctly on powerpc).

That type of a fix would probably be better handled in the slab allocator,
though, since kmalloc_node(nid) shouldn't crash just because nid is
memoryless, we should be doing local_memory_node(node) when allocating the
slab pages.

However, I don't think memoryless nodes are the problem here since Jiang
is testing for !node_online(nid) in his patch, so it's a problem with
cpu_to_node() pointing to an offline node. It makes sense for the page
allocator to crash in such a case, the node id is erroneous.

So either the cpu-to-node mapping is invalid or alloc_fair_sched_group()
is allocating memory for a cpu on an offline node. The
for_each_possible_cpu() looks suspicious. There's no guarantee that
local_memory_node(node) for an offline node will return anything with
affinity, so falling back to NUMA_NO_NODE looks appropriate in Jiang's
patch.
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