Linux Kernel Architecture

Chapter3:MemoryManagement

mm/highmem.c

void *page_address(struct page *page)

Figure 3-41 outlines the interplay between the above data structures.

struct page_address_map

page_address_map->virtual

page_address_map->page

LAST_

PKMAP

pkmap_count

page_address_htable

mem_map

PKMAP_

BASE

LAST_

PKMAP

FIXMAP_

START

Virtual address space

0

0

1

2

12

Figure 3-41: Data structures for managing persistent mappings.

Finding Page Addresses

page_addressfirst checks whether the passedpageinstance is in normal memory or high memory. If the

former applies, the page address can be calculated from the position ofpagein themem_maparray. In the

latter case, the above hash table is referenced to find the virtual address.

Creating a Mapping

Thekmapfunction must be used to create a mapping by means of apagepointer.^24 It is only a front end

to establish whether the desired page really is in highmem. If not, the address yielded bypage_address

is returned as the result. Otherwise, the kernel delegates work tokmap_high, which is defined as follows:

mm/highmem.c

void fastcall *kmap_high(struct page *page)

{

unsigned long vaddr;

vaddr = (unsigned long)page_address(page);

if (!vaddr)

vaddr = map_new_virtual(page);

pkmap_count[PKMAP_NR(vaddr)]++;

return (void*) vaddr;

}

(^24) This function resides not only in arch/x86/mm/highmem_32.cbut also in include/asm-ppc/highmem.hand
include/asm-sparc/highmem.hwith practically the same definition.