kmap的實現分析
void *kmap(struct page *page)
{
might_sleep();
if (!PageHighMem(page)){//如果是低端內存,則直接返內存頁對應的直接映射虛擬地址
//printk("low mem page\n");
return page_address(page);//所有的低端內存,在內核初始化時就已經映射好了,並且是不變得,且物理到虛擬相差0xc0000000
}else{
//printk("high mem page\n");
}
return kmap_high(page);//高端內存頁
}
/**
* kmap_high - map a highmem page into memory
* @page: &struct page to map
*
* Returns the page's virtual memory address.
*
* We cannot call this from interrupts, as it may block.
*/
void *kmap_high(struct page *page)
{
unsigned long vaddr;
/*
* For highmem pages, we can't trust "virtual" until
* after we have the lock.
*/
lock_kmap();
vaddr = (unsigned long)page_address(page);
if (!vaddr)//如果該頁的映射還未建立
vaddr = map_new_virtual(page);//開始建立新的映射
pkmap_count[PKMAP_NR(vaddr)]++;//該數組的值爲1,說明映射已經建立,爲2表明該應聲存在着引用
BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 2);
unlock_kmap();
return (void*) vaddr;
}
static inline unsigned long map_new_virtual(struct page *page)
{
unsigned long vaddr;
int count;
start:
count = LAST_PKMAP; // 2MB/4096KB=512 entries = LAST_PKMAP
/* Find an empty entry */
for (;;) {
last_pkmap_nr = (last_pkmap_nr + 1) & LAST_PKMAP_MASK;
if (!last_pkmap_nr) {
flush_all_zero_pkmaps();
count = LAST_PKMAP;
}
if (!pkmap_count[last_pkmap_nr])//爲0,說明該虛擬地址不存在映射,沒人使用
break; /* Found a usable entry */
if (--count)//如果遍歷了整個kmap虛擬空間,都不能找到空閒的虛擬地址,則休眠等待unkmap釋放虛擬地址
continue;
/*
* Sleep for somebody else to unmap their entries
*/
{
DECLARE_WAITQUEUE(wait, current);
__set_current_state(TASK_UNINTERRUPTIBLE);
add_wait_queue(&pkmap_map_wait, &wait);
unlock_kmap();
schedule();
remove_wait_queue(&pkmap_map_wait, &wait);
lock_kmap();
/* Somebody else might have mapped it while we slept */
if (page_address(page))
return (unsigned long)page_address(page);
/* Re-start */
goto start;
}
}
vaddr = PKMAP_ADDR(last_pkmap_nr);//#define PKMAP_ADDR(nr) (PKMAP_BASE + ((nr) << PAGE_SHIFT))
set_pte_at(&init_mm, vaddr,
&(pkmap_page_table[last_pkmap_nr]), mk_pte(page, kmap_prot));
pkmap_count[last_pkmap_nr] = 1;
set_page_address(page, (void *)vaddr);
return vaddr;
}
上面代碼中的pkmap_page_table是kmap所對應的虛擬地址[PKMAP_BASE,PAGE_OFFSET]所對應的二級映射表,即pte table,該映射表剛好是4KB用來映射2MB的虛擬到物理地址
static void __init kmap_init(void)
{
#ifdef CONFIG_HIGHMEM
//獲取kmap所對應的虛擬地址[PKMAP_BASE,PAGE_OFFSET]所對應的二級映射表的開始地址。該二級映射表剛好就是一個物理頁的大小
pkmap_page_table = early_pte_alloc_and_install(pmd_off_k(PKMAP_BASE),
PKMAP_BASE, _PAGE_KERNEL_TABLE);
printk("************************************************\n");
printk("pkmap_page_table:%x, phy of pkmap_page_table:%x\n",pkmap_page_table,virt_to_phys(pkmap_page_table));
printk("************************************************\n");
#endif
}
上述函數中的pmd_off_k(PKMAP_BASE)是獲取PKMAP_BASE虛擬地址對應的一級映射表中所對應的頁表項地址,static pte_t * __init early_pte_alloc_and_install(pmd_t *pmd,
unsigned long addr, unsigned long prot)
{
if (pmd_none(*pmd)) {//如果一級頁表項無效,即還未分配該表項所指向二級頁表,即pte table
pte_t *pte = early_pte_alloc(pmd);//分配二級頁表,即pte tabble
early_pte_install(pmd, pte, prot);//將pte table的hw/pte page0,hw/pte page1分別填充到一級頁表項的低4byte和高4byte
}
BUG_ON(pmd_bad(*pmd));
return pte_offset_kernel(pmd, addr);//返回二級頁表中對應的頁表項地址。
}
以上過程,具體見下圖的映射關係圖1
kmap的實驗
#include <linux/module.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/types.h>
#include <linux/delay.h>
#include <linux/moduleparam.h>
#include <linux/slab.h>
#include <linux/errno.h>
#include <linux/ioctl.h>
#include <linux/cdev.h>
#include <linux/string.h>
#include <linux/list.h>
#include <linux/pci.h>
#include <linux/gpio.h>
#include <linux/gfp.h>
#include <asm/highmem.h>
下面的函數,是通過kmap分配高端內存頁,並且將分配得到的內存頁,都特別的設置成特殊的數據,依次爲:0x5a,0x5b,0x5cstruct page * map_high_mem(int order)
{
int i=0;
static int poison = 0x5a;
unsigned char *buf = NULL;
struct page *high_page = alloc_pages(__GFP_HIGHMEM,order); //指定可以從高端內存分配物理空閒頁
//struct page *high_page = alloc_pages(GFP_HIGHUSER,order);
if(high_page){
printk("high_page alloc success\n");
}else{
printk("high_page alloc failed\n");
}
buf = kmap(high_page);//爲該高端內存頁,建立臨時映射,該函數可能休眠
if(buf){
printk("kmap success,buf addr:%x\n",buf);//如果映射成功,返回影射後的虛擬地址
for(i=0;i<4096;i++)
buf[i] = poison;
poison++;
}else{
printk("kmap failed\n");
}
return high_page;
}
void free_high_mem(struct page *page,int order)
{
kunmap(page);//拆除映射
__free_pages(page,order);//釋放對應物理頁
}
struct page *page_array[5];
#define NUM_ORDER 0
static int __init dev_init(void)
{
int ret;
int i;
/*************************************************************/
i = 0;
page_array[i++] = map_high_mem(NUM_ORDER);//連續分配,映射三個物理頁
page_array[i++] = map_high_mem(NUM_ORDER);
page_array[i++] = map_high_mem(NUM_ORDER);
printk("module address,page_array:0x%x\n",page_array);
return ret;
}
static void __exit dev_exit(void)
{
int i = 0;
free_high_mem(page_array[i++],NUM_ORDER);
free_high_mem(page_array[i++],NUM_ORDER);
free_high_mem(page_array[i++],NUM_ORDER);
}
module_init(dev_init);
module_exit(dev_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("LKN@SCUT");
以上是我們的測試代碼,代碼編譯,加載執行。
[ 0.000000] pkmap_page_table:ef7fc000, phy of pkmap_page_table:2f7fc000(759MB)
[ 0.000000] ************************************************
[ 83.289132] kernel buffer virtial address:ee155000
[ 83.293936] kernel buffer physical address:2e155000
[ 83.298801] high_page alloc success
[ 83.302264] kmap success,buf addr:bfeee000------------->(a) //第一次kmap映射返回的虛擬地址,將該頁都初始化爲0x5a
[ 83.306393] high_page alloc success
[ 83.309809] kmap success,buf addr:bfeef000------------->(b) //第二次kmap映射返回的虛擬地址,將該頁都初始化爲0x5b
[ 83.313990] high_page alloc success
[ 83.317414] kmap success,buf addr:bfef0000------------->(c) //第三次kmap映射返回的虛擬地址,將該頁都初始化爲0x5c
2f7fc000 + 1024*2(800) + 238*4(3b8) = 2F7FCBB8(二級映射表項的物理地址)
case b: bfeef000---------->37b0c000
2f7fc000 + 1024*2(800) + 238*4(3bc) = 2F7FCBBC(二級映射表項的物理地址)
case c: bfef0000---------->37b0b000
2f7fc000 + 1024*2(800) + 240*4(3C0) = 2F7FCBC0(二級映射表項的物理地址)