Android 匿名共享内存初始化篇（三）

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Ashmem初始化流程从ashmem.c的__ashmem_init方法开始，代码如下：

835 static int __init ashmem_init(void)
836 {
837     int ret;
838 
839     ashmem_area_cachep = kmem_cache_create("ashmem_area_cache",
840                       sizeof(struct ashmem_area),
841                       0, 0, NULL);
842     if (unlikely(!ashmem_area_cachep)) {
843         pr_err("failed to create slab cache\n");
844         return -ENOMEM;
845     }
846 
847     ashmem_range_cachep = kmem_cache_create("ashmem_range_cache",
848                       sizeof(struct ashmem_range),
849                       0, 0, NULL);
850     if (unlikely(!ashmem_range_cachep)) {
851         pr_err("failed to create slab cache\n");
852         return -ENOMEM;
853     }
854 
855     ret = misc_register(&ashmem_misc);
856     if (unlikely(ret)) {
857         pr_err("failed to register misc device!\n");
858         return ret;
859     }
860 
861     register_shrinker(&ashmem_shrinker);
862 
863     pr_info("initialized\n");
864 
865     return 0;
866 }

第839行和第847行使用函数kmem_cache_create创建了两个slab缓冲分配器，ashmem_area_cachep和ashmem_range_cachep,前者用来分配ashmem_area结构体，后者用来分配ashmem_range结构体，第855行注册设备类型为misc，ashmem_misc设备结构体定义为：

829 static struct miscdevice ashmem_misc = {
830     .minor = MISC_DYNAMIC_MINOR,
831     .name = "ashmem",
832     .fops = &ashmem_fops,
833 };

根据该结构体定义可知Ashmem对应的设备文件为：/dev/ashmem，该结构体定义了fops字段，具体内容为ashmem_fops如下：

816 static const struct file_operations ashmem_fops = {
817     .owner = THIS_MODULE,
818     .open = ashmem_open,
819     .release = ashmem_release,
820     .read = ashmem_read,
821     .llseek = ashmem_llseek,
822     .mmap = ashmem_mmap,
823     .unlocked_ioctl = ashmem_ioctl,
824 #ifdef CONFIG_COMPAT
825     .compat_ioctl = compat_ashmem_ioctl,
826 #endif
827 };

根据字段可以看出该结构体定义了对Ashmem的打开、释放、读取、映射、seek等操作。

继续看__ashmem_init函数的第861行，调用register_shrinker函数向内存管理系统注册了一个内存回收函数ashmem_shrinker，当系统内存不足的时候就会调用该函数释放内存。