Hello, Christoph.

On Mon, Jan 30, 2012 at 11:52:23AM -0600, Christoph Lameter wrote:

We have two possibilities now:

1. We say that the value returned from the per cpu allocator is an opaque
value.

	This means that we have to remove the NULL check from the free
	function. And audit the kernel code for all occurrences where
	a per cpu pointer value of NULL is assumed to mean that no per
	cpu allocation has occurred.

No, NULL is never gonna be a valid return from any allocator including
percpu.  Percpu allocator doesn't and will never do so.

2. We say that there are special values for the per cpu pointers (NULL,
	ZERO_SIZE_PTR)

	Then we would have to guarantee that the per cpu allocator never
	returns those values.

	Plus then the ZERO_SIZE_PTR patch will be fine.

	The danger exist of these values being passed as
	parameters to functions that do not support them (per_cpu_ptr
	etc). Those would need VM_BUG_ONs or some other checks to detect
	potential problems.

I'm saying we don't have this for ZERO_SIZE_PTR in any meaningful way
at this point.  If somebody wants to implement it properly, please
feel free to, but simply applying ZERO_SIZE_PTR without other changes
doesn't make any sense.

Thanks.

-- 
tejun

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