Re: [PATCH 1/3] slqb: Do not use DEFINE_PER_CPU for per-node data

[Mel Gorman <mel@csn.ul.ie>]

On Mon, Sep 21, 2009 at 09:24:00AM +0300, Pekka Enberg wrote:
> Hi Tejun,
> 
> On Mon, 2009-09-21 at 00:55 +0900, Tejun Heo wrote:
> > Pekka Enberg wrote:
> > > Tejun Heo wrote:
> > >> Pekka Enberg wrote:
> > >>> On Fri, Sep 18, 2009 at 10:34 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> > >>>> SLQB used a seemingly nice hack to allocate per-node data for the
> > >>>> statically
> > >>>> initialised caches. Unfortunately, due to some unknown per-cpu
> > >>>> optimisation, these regions are being reused by something else as the
> > >>>> per-node data is getting randomly scrambled. This patch fixes the
> > >>>> problem but it's not fully understood *why* it fixes the problem at the
> > >>>> moment.
> > >>> Ouch, that sounds bad. I guess it's architecture specific bug as x86
> > >>> works ok? Lets CC Tejun.
> > >>
> > >> Is the corruption being seen on ppc or s390?
> > > 
> > > On ppc.
> > 
> > Can you please post full dmesg showing the corruption?  Also, if you
> > apply the attached patch, does the added BUG_ON() trigger?
> 
> I don't have the affected machines, Sachin and Mel do.
> 

More accurately, Sachin does but he gave me access to the machine for
this bug so I can use it when it's otherwise idle.

> 			Pekka
> 
> > diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> > index 878836c..fb690d2 100644
> > --- a/include/linux/percpu.h
> > +++ b/include/linux/percpu.h
> > @@ -127,7 +127,7 @@ extern int __init pcpu_page_first_chunk(size_t reserved_size,
> >   * dynamically allocated. Non-atomic access to the current CPU's
> >   * version should probably be combined with get_cpu()/put_cpu().
> >   */
> > -#define per_cpu_ptr(ptr, cpu)	SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu)))
> > +#define per_cpu_ptr(ptr, cpu)	({ BUG_ON(!(ptr)); SHIFT_PERCPU_PTR((ptr), per_cpu_offset((cpu))); })
> > 
> >  extern void *__alloc_reserved_percpu(size_t size, size_t align);
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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