Re: [PATCH 4.4 66/90] powerpc/numa: Fix percpu allocations to be NUMA aware

[Michal Hocko]

On Fri 28-07-17 15:41:47, Greg KH wrote:
> On Fri, Jul 28, 2017 at 03:53:35PM +0200, Michal Hocko wrote:
> > JFYI. We have encountered a regression after applying this patch on a
> > large ppc machine. While the patch is the right thing to do it doesn't
> > work well with the current vmalloc area size on ppc and large machines
> > where NUMA nodes are very far from each other. Just for the reference
> > the boot fails on such a machine with bunch of warning preceeding it.
> > See http://lkml.kernel.org/r/20170724134240.GL25221@xxxxxxxxxxxxxx
> > 
> > It seems the right thing to do is to enlarge the vmalloc space on ppc
> > but this is not the case in the upstream kernel yet AFAIK. It is also
> > questionable whether that is a stable material but I will decision on
> > you here.
> > 
> > We have reverted this patch from our 4.4 based kernel.
> 
> But all is fine on newer kernels?  That is odd.

Newer kernels do not have enlarged vmalloc space yet AFAIK so they won't
work properly eiter. This bug is quite rare though because you need a
specific HW configuration to trigger the issue - namely NUMA nodes have
to be far away from each other in the physical memory space.

> I'll be glad to drop it, but should it be dropped from all stable trees?

Yes from all stable backports.
-- 
Michal Hocko
SUSE Labs