Re: [PATCH for v3.17-rc1] Revert "slab: remove BAD_ALIEN_MAGIC"

[Christoph Lameter]

On Fri, 8 Aug 2014, Geert Uytterhoeven wrote:

> On Fri, Aug 8, 2014 at 4:44 PM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> > On Fri, 8 Aug 2014, Joonsoo Kim wrote:
> >
> >> This reverts commit a640616822b2 ("slab: remove BAD_ALIEN_MAGIC").
> >
> > Lets hold off on this one. I am bit confused as to why a non NUMA system
> > would have multiple NUMA nodes.
>
> DISCONTIGMEM
>
> mm/Kconfig:
>
> #
> # Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
> # to represent different areas of memory.  This variable allows
> # those dependencies to exist individually.
> #
> config NEED_MULTIPLE_NODES
>         def_bool y
>         depends on DISCONTIGMEM || NUMA

Uhhh... And how does one access memory when the node is != 0 given that
zone_to_nid always returns 0 in the !CONFIG_NUMA case? AFAICT there are
numerous of these node == 0 assumptions in the kernel for !NUMA.

include/linux/mmzone.h:

#ifdef CONFIG_NUMA
#define pfn_to_nid(pfn)                                                 \
({                                                                      \
        unsigned long __pfn_to_nid_pfn = (pfn);                         \
        page_to_nid(pfn_to_page(__pfn_to_nid_pfn));                     \
})
#else
#define pfn_to_nid(pfn)         (0)
#endif


How can this work at all????




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