内核和用户空间共享内存的实现例程-proc和mmap

之所以想写这篇帖子，是有两个方面原因。其一是内核版有一个关于《内核可以从线性地址直接计算物理地址，用来做什么呢？》的讨论，偶说计算出物理地址可以共享给用户空间读写。dreamice兄说能否说一下详细的应用。其二是alb*版主提到wheelz曾经写过这样一个例程，拜读了一把，发现在传递物理地址和内存大小上，wheelz的例程还有些不够灵活。alb*版主提到可以通过文件的方式实现动态的传递。

因此，偶也写了这样一个例程，可以动态的将内核空间的物理地址和大小传给用户空间。

整个内核模块，在模块插入时建立proc文件，分配内存。卸载模块的时候将用户空间写入的内容打印出来。

以下是内核模块的代码和用户空间的测试代码。

/*This program is used to allocate memory in kernel
and pass the physical address to userspace through proc file.*/

#include <linux/version.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/mm.h>

#define PROC_MEMSHARE_DIR                                "memshare"
#define PROC_MEMSHARE_PHYADDR                        "phymem_addr"
#define PROC_MEMSHARE_SIZE                                "phymem_size"

/*alloc one page. 4096 bytes*/
#define PAGE_ORDER                                0
/*this value can get from PAGE_ORDER*/
#define PAGES_NUMBER                                1

struct proc_dir_entry *proc_memshare_dir ;
unsigned long kernel_memaddr = 0;
unsigned long kernel_memsize= 0;

static int proc_read_phymem_addr(char *page, char **start, off_t off, int count)
{
return sprintf(page, "%08lx\n", __pa(kernel_memaddr));
}
static int proc_read_phymem_size(char *page, char **start, off_t off, int count)
{
return sprintf(page, "%lu\n", kernel_memsize);
}

static int __init init(void)
{
/*build proc dir "memshare"and two proc files: phymem_addr, phymem_size in the dir*/
proc_memshare_dir = proc_mkdir(PROC_MEMSHARE_DIR, NULL);
create_proc_info_entry(PROC_MEMSHARE_PHYADDR, 0, proc_memshare_dir, proc_read_phymem_addr);
create_proc_info_entry(PROC_MEMSHARE_SIZE, 0, proc_memshare_dir, proc_read_phymem_size);

/*alloc one page*/
kernel_memaddr =__get_free_pages(GFP_KERNEL, PAGE_ORDER);
if(!kernel_memaddr)
{
printk("Allocate memory failure!\n");
}
else
{
SetPageReserved(virt_to_page(kernel_memaddr));
kernel_memsize = PAGES_NUMBER * PAGE_SIZE;
printk("Allocate memory success!. The phy mem addr=%08lx, size=%lu\n", __pa(kernel_memaddr), kernel_memsize);
}
return 0;
}

static void __exit fini(void)
{
printk("The content written by user is: %s\n", (unsigned char *) kernel_memaddr);
ClearPageReserved(virt_to_page(kernel_memaddr));
free_pages(kernel_memaddr, PAGE_ORDER);
remove_proc_entry(PROC_MEMSHARE_PHYADDR, proc_memshare_dir);
remove_proc_entry(PROC_MEMSHARE_SIZE, proc_memshare_dir);
remove_proc_entry(PROC_MEMSHARE_DIR, NULL);

return;
}
module_init(init);
module_exit(fini);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Godbach (nylzhaowei@163.com)");
MODULE_DESCRIPTION("Kernel memory share module.");

用户空间的测试代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/mman.h>

int main(int argc, char* argv[])
{
if(argc != 2)
{
printf("Usage: %s string\n", argv[0]);
return 0;
}

unsigned long phymem_addr, phymem_size;
char *map_addr;
char s[256];
int fd;

/*get the physical address of allocated memory in kernel*/
fd = open("/proc/memshare/phymem_addr", O_RDONLY);
if(fd < 0)
{
printf("cannot open file /proc/memshare/phymem_addr\n");
return 0;
}
read(fd, s, sizeof(s));
sscanf(s, "%lx", &phymem_addr);
close(fd);

/*get the size of allocated memory in kernel*/
fd = open("/proc/memshare/phymem_size", O_RDONLY);
if(fd < 0)
{
printf("cannot open file /proc/memshare/phymem_size\n");
return 0;
}
read(fd, s, sizeof(s));
sscanf(s, "%lu", &phymem_size);
close(fd);

printf("phymem_addr=%lx, phymem_size=%lu\n", phymem_addr, phymem_size);
/*memory map*/
int map_fd = open("/dev/mem", O_RDWR);
if(map_fd < 0)
{
printf("cannot open file /dev/mem\n");
return 0;
}

map_addr = mmap(0, phymem_size, PROT_READ|PROT_WRITE, MAP_SHARED, map_fd, phymem_addr);
strcpy(map_addr, argv[1]);
munmap(map_addr, phymem_size);
close(map_fd);
return 0;

}

测试的内核是2.6.25.以下是执行结果。

debian:/home/km/memshare# insmod memshare_kernel.ko

debian:/home/km/memshare# ./memshare_user 'hello,world!'

phymem_addr=e64e000, phymem_size=4096

debian:/home/km/memshare# cat /proc/memshare/phymem_addr

0e64e000

debian:/home/km/memshare# cat /proc/memshare/phymem_size

4096

debian:/home/km/memshare# rmmod memshare_kernel

debian:/home/km/memshare# tail /var/log/messages

Sep 27 18:14:24 debian kernel: [50527.567931] Allocate memory success!. The phy mem addr=0e64e000, size=4096

Sep 27 18:15:31 debian kernel: [50592.570986] The content written by user is: hello,world!

仓促之间，有些地方处理的还是比较简单。希望高手多多指正。需要了解这方面实现的可以参考一下，共同学习。