[PATCH] percpu: add comment to per_cpu_ptr_to_phys

[Dave Young <dyoung@redhat.com>]

add comments about current per_cpu_ptr_to_phys implementation
to explain why the logic is more complicated than necessary.

Signed-off-by: Dave Young <dyoung@redhat.com>
---
 mm/percpu.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

--- linux-2.6.orig/mm/percpu.c	2011-11-22 10:18:46.000000000 +0800
+++ linux-2.6/mm/percpu.c	2011-11-23 16:27:01.667562973 +0800
@@ -988,6 +988,19 @@ phys_addr_t per_cpu_ptr_to_phys(void *ad
 	unsigned int cpu;
 
 	/*
+	 * percpu allocator has special setup for the first chunk,
+	 * which currently supports either embedding in linear address space
+	 * or vmalloc mapping, and, from the second one, the backing
+	 * allocator (currently either vm or km) provides translation.
+	 *
+	 * The addr can be tranlated simply without checking if it falls
+	 * into the first chunk. But the current code reflects better
+	 * how percpu allocator actually works, and the verification can
+	 * discover both bugs in percpu allocator itself and
+	 * per_cpu_ptr_to_phys() callers. So we keep current code.
+	 */
+
+	/*
 	 * The following test on first_start/end isn't strictly
 	 * necessary but will speed up lookups of addresses which
 	 * aren't in the first chunk.