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.
Signed-off-by: Anshuman Khandual <khand...@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 _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
