各种内存分配函数的比较

通过下面的程序来测试各种内核内存分配函数的不同。

#include <linux/init.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/slab.h>
#include <linux/gfp.h>
#include <linux/mm.h>
MODULE_LICENSE("Dual BSD/GPL");

void *k_DMA = NULL, *k_KERN = NULL, *k_HM = NULL,
    *gfp_DMA = NULL, *gfp_KERN = NULL, *gfp_HM = NULL,
    *v = NULL;
struct page *ap_DMA = NULL, *ap_KERN = NULL, *ap_HM = NULL;

static int main_init(void)
{
    k_DMA = kmalloc(4096, GFP_KERNEL | __GFP_DMA);
    k_KERN = kmalloc(4096, GFP_KERNEL);
    k_HM = kmalloc(4096, GFP_KERNEL | __GFP_HIGHMEM);
        printk(KERN_INFO "kmalloc(DMA) address is 0x%x\n", k_DMA);
    printk(KERN_INFO "kmalloc(KERN) address is 0x%x\n", k_KERN);
    printk(KERN_INFO "kmalloc(HM) address is 0x%x\n", k_HM);


        gfp_DMA = __get_free_pages(GFP_KERNEL | __GFP_DMA, 1);
    gfp_KERN = __get_free_pages(GFP_KERNEL, 1);
    // oops！
    //gfp_HM = __get_free_pages(GFP_KERNEL | __GFP_HIGHMEM, 1);
    printk(KERN_INFO "get_free_pages(DMA) address is 0x%x\n", gfp_DMA);
    printk(KERN_INFO "get_free_pages(KERN) address is 0x%x\n", gfp_KERN);

    ap_DMA = alloc_pages(GFP_KERNEL | __GFP_DMA, 1);
    ap_KERN = alloc_pages(GFP_KERNEL, 1);
    ap_HM = alloc_pages(GFP_KERNEL | __GFP_HIGHMEM, 1);
    printk(KERN_INFO "alloc_pages(DMA) address is 0x%x\n", page_address(ap_DMA));
    printk(KERN_INFO "alloc_pages(KERN) address is 0x%x\n", page_address(ap_KERN));
    printk(KERN_INFO "alloc_pages(HM) address is 0x%x\n", page_address(ap_HM));

    v = vmalloc(PAGE_SIZE);
    printk(KERN_INFO "vmallo address is 0x%x\n", v);

    return 0;
}

static void main_exit(void)
{
    vfree(v);
    __free_pages(ap_HM, 1);
    __free_pages(ap_KERN, 1);
    __free_pages(ap_DMA, 1);
    free_pages(gfp_KERN, 1);
    free_pages(gfp_DMA, 1);
    kfree(k_HM);
    kfree(k_KERN);
    kfree(k_DMA);
}

module_init(main_init);
module_exit(main_exit);

程序的输出为：
[  122.429134] kmalloc(DMA) address is 0xc0e08000
[  122.429143] kmalloc(KERN) address is 0xf1570000
[  122.429149] kmalloc(HM) address is 0xf1575000

[  122.429158] get_free_pages(DMA) address is 0xc0e02000
[  122.429163] get_free_pages(KERN) address is 0xef37e000

[  122.429173] alloc_pages(DMA) address is 0xc0e04000
[  122.429178] alloc_pages(KERN) address is 0xef160000
[  122.429184] alloc_pages(HM) address is 0x0

[  122.429194] vmallo address is 0xf7e69000

由程序的输出可以看出，kmalloc不能分配高端内存，get_free_pages也是，并且后者如果指定了__GFP_HIGHMEM标志还会产生oops。
alloc_pages可以分配高端内存，当分配的内存页位于高端内存时，page_address函数就无法获得这个内存页的逻辑地址了（page_address实际上是返回page结构的virtual域），因为内存页和内核地址空间不是简单的线性关系了。
kmalloc，get_free_pages和alloc_pages返回的都是连续的物理空间。
vmalloc分配的内存的虚拟地址位于内核地址空间的高128M。
get_free_pages和alloc_pages的分配结果会反映在/proc/buddyinfo文件中。