[RFC 2/4] powerpc/mm: Add comments to the vmemmap layout

[Aneesh Kumar K.V]

Anshuman Khandual <khandual at linux.vnet.ibm.com> writes:

> 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 at linux.vnet.ibm.com>


> Signed-off-by: Anshuman Khandual <khandual at linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/book3s/64/pgtable.h | 41 ++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 8d1c41d..9db4a86 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -26,6 +26,47 @@
>  #define IOREMAP_BASE	(PHB_IO_END)
>  #define IOREMAP_END	(KERN_VIRT_START + KERN_VIRT_SIZE)
>
> +/*
> + * Starting address of the virtual address space where all page structs
> + * for the system physical memory are stored under the vmemmap sparse
> + * memory model. All possible struct pages are logically stored in a
> + * sequence in this virtual address space irrespective of the fact
> + * whether any given PFN is valid or even the memory section is valid
> + * or not. During boot and memory hotplug add operation when new memory
> + * sections are added, real physical allocation and hash table bolting
> + * will be performed. This saves precious physical memory when the system
> + * really does not have valid PFNs in some address ranges.
> + *
> + *  vmemmap +--------------+
> + *     +    |  page struct +----------+  PFN is valid
> + *     |    +--------------+          |
> + *     |    |  page struct |          |  PFN is invalid
> + *     |    +--------------+          |
> + *     |    |  page struct +------+   |
> + *     |    +--------------+      |   |
> + *     |    |  page struct |      |   |
> + *     |    +--------------+      |   |
> + *     |    |  page struct |      |   |
> + *     |    +--------------+      |   |
> + *     |    |  page struct +--+   |   |
> + *     |    +--------------+  |   |   |
> + *     |    |  page struct |  |   |   |       +-------------+
> + *     |    +--------------+  |   |   +-----> |     PFN     |
> + *     |    |  page struct |  |   |           +-------------+
> + *     |    +--------------+  |   +---------> |     PFN     |
> + *     |    |  page struct |  |               +-------------+
> + *     |    +--------------+  +-------------> |     PFN     |
> + *     |    |  page struct |                  +-------------+
> + *     |    +--------------+           +----> |     PFN     |
> + *     |    |  page struct |           |      +-------------+
> + *     |    +--------------+           |    Bolted in hash table
> + *     |    |  page struct +-----------+
> + *     v    +--------------+
> + *
> + * VMEMMAP_BASE (0xf000000000000000) region has a total range of 64TB but
> + * then it uses NR_MEM_SECTIONS * PAGES_PER_SECTION * sizeof(page struct)
> + * amount of virtual memory from it.
> + */
>  #define vmemmap			((struct page *)VMEMMAP_BASE)
>
>  /* Advertise special mapping type for AGP */
> -- 
> 2.1.0