powerpc/mm: Add comments to the vmemmap layout

Add some explaination to the layout of vmemmap virtual address
space and how physical page mapping is only used for valid PFNs
present at any point on the system.
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pgtable-hash64.c

@@ -21,6 +21,81 @@
 #include <trace/events/thp.h>
 
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
+/*
+ * vmemmap is the starting address of the virtual address space where
+ * struct pages are allocated for all possible PFNs present on the system
+ * including holes and bad memory (hence sparse). These virtual struct
+ * pages are stored in sequence in this virtual address space irrespective
+ * of the fact whether the corresponding PFN is valid or not. This achieves
+ * constant relationship between address of struct page and its PFN.
+ *
+ * During boot or memory hotplug operation when a new memory section is
+ * added, physical memory allocation (including hash table bolting) will
+ * be performed for the set of struct pages which are part of the memory
+ * section. This saves memory by not allocating struct pages for PFNs
+ * which are not valid.
+ *
+ *		----------------------------------------------
+ *		| PHYSICAL ALLOCATION OF VIRTUAL STRUCT PAGES|
+ *		----------------------------------------------
+ *
+ *	   f000000000000000                  c000000000000000
+ * vmemmap +--------------+                  +--------------+
+ *  +      |  page struct | +--------------> |  page struct |
+ *  |      +--------------+                  +--------------+
+ *  |      |  page struct | +--------------> |  page struct |
+ *  |      +--------------+ |                +--------------+
+ *  |      |  page struct | +       +------> |  page struct |
+ *  |      +--------------+         |        +--------------+
+ *  |      |  page struct |         |   +--> |  page struct |
+ *  |      +--------------+         |   |    +--------------+
+ *  |      |  page struct |         |   |
+ *  |      +--------------+         |   |
+ *  |      |  page struct |         |   |
+ *  |      +--------------+         |   |
+ *  |      |  page struct |         |   |
+ *  |      +--------------+         |   |
+ *  |      |  page struct |         |   |
+ *  |      +--------------+         |   |
+ *  |      |  page struct | +-------+   |
+ *  |      +--------------+             |
+ *  |      |  page struct | +-----------+
+ *  |      +--------------+
+ *  |      |  page struct | No mapping
+ *  |      +--------------+
+ *  |      |  page struct | No mapping
+ *  v      +--------------+
+ *
+ *		-----------------------------------------
+ *		| RELATION BETWEEN STRUCT PAGES AND PFNS|
+ *		-----------------------------------------
+ *
+ * vmemmap +--------------+                 +---------------+
+ *  +      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |              |
+ *  |      +--------------+
+ *  |      |              |
+ *  |      +--------------+
+ *  |      |              |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |              |
+ *  |      +--------------+
+ *  |      |              |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  |      +--------------+                 +---------------+
+ *  |      |  page struct | +-------------> |      PFN      |
+ *  v      +--------------+                 +---------------+
+ */
 /*
  * On hash-based CPUs, the vmemmap is bolted in the hash table.
  *