On Mon, 30 Jan 2012, Tejun Heo wrote:

I'm pretty sure it never gives out NULL for a dynamic allocation.  The
base might be mapped to zero but we're guaranteed to have some static
percpu areas there and IIRC the percpu addresses aren't supposed to
wrap.

True but there is a check for a NULL pointer on free. So a NULL pointer
currently has the semantics of being an unallocated per cpu structure.
If the allocator returns NULL by accident then we cannot free the per cpu
allocation anymore.

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