《Linux内核设计与实现》读书笔记（十二）- 内存管理

/*
 * 页中包含的成员非常多，还包含了一些联合体
 * 其中有些字段我暂时还不清楚含义，以后再补上。。。
 */
struct page {
    unsigned long flags;    /* 存放页的状态，各种状态参见<linux/page-flags.h> */
    atomic_t _count;        /* 页的引用计数 */
    union {
        atomic_t _mapcount;    /* 已经映射到mms的pte的个数 */
        struct {        /* 用于slab层 */
            u16 inuse;
            u16 objects;
        };
    };
    union {
        struct {
        unsigned long private;        /* 此page作为私有数据时，指向私有数据 */
        struct address_space *mapping;    /* 此page作为页缓存时，指向关联的address_space */
        };
#if USE_SPLIT_PTLOCKS
        spinlock_t ptl;
#endif
        struct kmem_cache *slab;    /* 指向slab层 */
        struct page *first_page;    /* 尾部复合页中的第一个页 */
    };
    union {
        pgoff_t index;        /* Our offset within mapping. */
        void *freelist;        /* SLUB: freelist req. slab lock */
    };
    struct list_head lru;    /* 将页关联起来的链表项 */
#if defined(WANT_PAGE_VIRTUAL)
    void *virtual;            /* 页的虚拟地址 */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
    unsigned long debug_flags;    /* Use atomic bitops on this */
#endif

#ifdef CONFIG_KMEMCHECK
    /*
     * kmemcheck wants to track the status of each byte in a page; this
     * is a pointer to such a status block. NULL if not tracked.
     */
    void *shadow;
#endif
};

/**
 * @size  - 申请分配的字节数
 * @flags - 上面讨论的各种 gfp_mask
 */
static __always_inline void *kmalloc(size_t size, gfp_t flags)
#+end_src

vmalloc的定义在 mm/vmalloc.c 中
#+begin_src C
/**
 * @size - 申请分配的字节数
 */
void *vmalloc(unsigned long size)

#include <linux/slab.h>
#include <linux/slab_def.h>
#include "kn_common.h"

MODULE_LICENSE("Dual BSD/GPL");

#define MYSLAB "testslab"

static struct kmem_cache *myslab;

/* 申请内存时调用的构造函数 */
static void ctor(void* obj)
{
    printk(KERN_ALERT "constructor is running....\n");
}

struct student
{
    int id;
    char* name;
};

static void print_student(struct student *);


static int testslab_init(void)
{
    struct student *stu1, *stu2;
    
    /* 建立slab高速缓存，名称就是宏 MYSLAB */
    myslab = kmem_cache_create(MYSLAB,
                               sizeof(struct student),
                               0,
                               0,
                               ctor);

    /* 高速缓存中分配2个对象 */
    printk(KERN_ALERT "alloc one student....\n");
    stu1 = (struct student*)kmem_cache_alloc(myslab, GFP_KERNEL);
    stu1->id = 1;
    stu1->name = "wyb1";
    print_student(stu1);
    
    printk(KERN_ALERT "alloc one student....\n");
    stu2 = (struct student*)kmem_cache_alloc(myslab, GFP_KERNEL);
    stu2->id = 2;
    stu2->name = "wyb2";
    print_student(stu2);
    
    /* 释放高速缓存中的对象 */
    printk(KERN_ALERT "free one student....\n");
    kmem_cache_free(myslab, stu1);

    printk(KERN_ALERT "free one student....\n");
    kmem_cache_free(myslab, stu2);

    /* 执行完后查看 /proc/slabinfo 文件中是否有名称为 “testslab”的缓存 */
    return 0;
}

static void testslab_exit(void)
{
    /* 删除建立的高速缓存 */
    printk(KERN_ALERT "*************************\n");
    print_current_time(0);
    kmem_cache_destroy(myslab);
    printk(KERN_ALERT "testslab is exited!\n");
    printk(KERN_ALERT "*************************\n");

    /* 执行完后查看 /proc/slabinfo 文件中是否有名称为 “testslab”的缓存 */
}

static void print_student(struct student *stu)
{
    if (stu != NULL)
    {
        printk(KERN_ALERT "**********student info***********\n");
        printk(KERN_ALERT "student id   is: %d\n", stu->id);
        printk(KERN_ALERT "student name is: %s\n", stu->name);
        printk(KERN_ALERT "*********************************\n");
    }
    else
        printk(KERN_ALERT "the student info is null!!\n");    
}

module_init(testslab_init);
module_exit(testslab_exit);

# must complile on customize kernel
obj-m += myslab.o
myslab-objs := testslab.o kn_common.o

#generate the path
CURRENT_PATH:=$(shell pwd)
#the current kernel version number
LINUX_KERNEL:=$(shell uname -r)
#the absolute path
LINUX_KERNEL_PATH:=/usr/src/kernels/$(LINUX_KERNEL)
#complie object
all:
    make -C $(LINUX_KERNEL_PATH) M=$(CURRENT_PATH) modules
    rm -rf modules.order Module.symvers .*.cmd *.o *.mod.c .tmp_versions *.unsigned
#clean
clean:
    rm -rf modules.order Module.symvers .*.cmd *.o *.mod.c *.ko .tmp_versions *.unsigned

[root@vbox chap12]# make
[root@vbox chap12]# insmod myslab.ko 
[root@vbox chap12]# dmesg | tail -220 
# 可以看到第一次申请内存时，系统一次分配很多内存用于缓存(构造函数执行了多次)
[root@vbox chap12]# cat /proc/slabinfo | grep test #查看我们建立的缓存名在不在系统中
testslab               0      0     16  202    1 : tunables  120   60    0 : slabdata      0      0      0
[root@vbox chap12]# rmmod myslab.ko #卸载内核模块
[root@vbox chap12]# cat /proc/slabinfo | grep test #我们的缓存名已经不在系统中了