根文件系统挂载

------------------------------------ 本文系本站原创,欢迎转载! 转载请注明出处:http://sjj0412.cublog.cn/ ------------------------------------------ 由bootload进入linux后由head.s进入了start_kernel了. asmlinkage void __init start_kernel(void) { char * command_line; extern struct kernel_param __start___param[], __stop___param[]; ……………….. setup_arch(&command_line); …………. ……….. vfs_caches_init(num_physpages); …………. …………... rest_init(); } 重要函数解释 1.Setup_arch是解释bootloader传过来的参数,并附相关参数。 void __init setup_arch(char **cmdline_p) { struct tag *tags = (struct tag *)&init_tags; struct machine_desc *mdesc; char *from = default_command_line; setup_processor(); mdesc = setup_machine(machine_arch_type); machine_name = mdesc->name; if (mdesc->soft_reboot) reboot_setup("s"); if (mdesc->boot_params) tags = phys_to_virt(mdesc->boot_params); /* * If we have the old style parameters, convert them to * a tag list. */ if (tags->hdr.tag != ATAG_CORE) convert_to_tag_list(tags); if (tags->hdr.tag != ATAG_CORE) tags = (struct tag *)&init_tags; if (mdesc->fixup) mdesc->fixup(mdesc, tags, &from, &meminfo); if (tags->hdr.tag == ATAG_CORE) { if (meminfo.nr_banks != 0) squash_mem_tags(tags); parse_tags(tags); } init_mm.start_code = (unsigned long) &_text; init_mm.end_code = (unsigned long) &_etext; init_mm.end_data = (unsigned long) &_edata; init_mm.brk = (unsigned long) &_end; memcpy(saved_command_line, from, COMMAND_LINE_SIZE); saved_command_line[COMMAND_LINE_SIZE-1] = '/0'; parse_cmdline(cmdline_p, from); paging_init(&meminfo, mdesc); request_standard_resources(&meminfo, mdesc); cpu_init(); /* * Set up various architecture-specific pointers */ init_arch_irq = mdesc->init_irq; system_timer = mdesc->timer; init_machine = mdesc->init_machine; #ifdef CONFIG_VT #if defined(CONFIG_VGA_CONSOLE) conswitchp = &vga_con; #elif defined(CONFIG_DUMMY_CONSOLE) conswitchp = &dummy_con; #endif #endif } 到这里就不得不说__setup宏。 __setup宏来注册关键字及相关联的处理函数,__setup宏在include/linux/init.h中定义,其原型如下: __setup(string, function_handler) 其中:string是关键字,function_handler是关联处理函数。__setup只是告诉内核在启动时输入串中含有string时,内核要去执行function_handler。String必须以“=”符结束以使parse_args更方便解析。紧随“=”后的任何文本都会作为输入传给 function_handler。 下面的例子来自于init/do_mounts.c,其中root_dev_setup作为处理程序被注册给“root=”关键字: __setup("root=", root_dev_setup); 比如我们在启动向参数终有 noinitrd root=/dev/mtdblock2 console=/linuxrc setup_arch解释时会发现root=/dev/mtdblock2,然后它就会调用root_dev_setup static int __init root_dev_setup(char *line) { strlcpy(saved_root_name, line, sizeof(saved_root_name)); return 1; } 我们容易看出,这个函数就是给saved_root_name赋值,saved_root_name这个全局变量非常重要,在后面的根目录加载还会讲到。 同理,其他参数也会做相应处理。 2. vfs_caches_init void __init vfs_caches_init(unsigned long mempages) { unsigned long reserve; /* Base hash sizes on available memory, with a reserve equal to 150% of current kernel size */ reserve = min((mempages - nr_free_pages()) * 3/2, mempages - 1); mempages -= reserve; names_cachep = kmem_cache_create("names_cache", PATH_MAX, 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL, NULL); filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, filp_ctor, filp_dtor); dcache_init(mempages); inode_init(mempages); files_init(mempages); mnt_init(mempages); bdev_cache_init(); chrdev_init(); } Mnt_init会创建一个rootfs,这个是虚拟的rootfs,是内存文件系统(和ramfs),后面还会指向具体的根文件系统。 void __init mnt_init(unsigned long mempages) { struct list_head *d; unsigned int nr_hash; int i; mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct vfsmount), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL, NULL); sysfs_init(); init_rootfs();//注册rootfs文件系统。 init_mount_tree();//创建rootfs文件系统 } static void __init init_mount_tree(void) { struct vfsmount *mnt; struct namespace *namespace; struct task_struct *g, *p; mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);//创建了文件系统 if (IS_ERR(mnt)) panic("Can't create rootfs"); namespace = kmalloc(sizeof(*namespace), GFP_KERNEL); if (!namespace) panic("Can't allocate initial namespace"); atomic_set(&namespace->count, 1); INIT_LIST_HEAD(&namespace->list); init_rwsem(&namespace->sem); list_add(&mnt->mnt_list, &namespace->list); namespace->root = mnt; mnt->mnt_namespace = namespace; init_task.namespace = namespace; read_lock(&tasklist_lock); do_each_thread(g, p) { get_namespace(namespace); p->namespace = namespace; } while_each_thread(g, p); read_unlock(&tasklist_lock); set_fs_pwd(current->fs, namespace->root, namespace->root->mnt_root); set_fs_root(current->fs, namespace->root, namespace->root->mnt_root);//将rootfs设为根文件系统,这里只是暂时的。 } 大家可能会会说为什么要这样一个过程了,为什么不直接将我们设置的root=/dev/mtdblock2设备做为根文件系统挂载啊。 首先有两方面的原因 1.可能内核中没有根文件系统设备的驱动(usb,sata硬盘的驱动),需要像initrd,initramdisk,cpio-initrd安装驱动,然后再去根文件系统设备取数据然后挂载,而这些往往需要以文件系统格式访问,故首先需要建立文件系统。 2.因为我们的root设备往往以设备文件的形式给出,如果没有文件系统,怎么会有设备文件之说呢,内核怎么知道如何访问根文件系统设备,这就是鸡蛋和鸡的问题,也许有人又说哪这个虚拟的根文件系统的设备文件在哪。其实由于其是虚拟的,叫内存文件系统,也就 是人为的给它一个设备号(0,255),人为的创建内存根目录。 3.rest_init static void noinline rest_init(void) __releases(kernel_lock) { kernel_thread(init, NULL, CLONE_FS | CLONE_SIGHAND);//大名鼎鼎的init进程 numa_default_policy(); unlock_kernel(); preempt_enable_no_resched(); /* * The boot idle thread must execute schedule() * at least one to get things moving: */ schedule(); cpu_idle(); } 下面我们就来Init函数 static int init(void * unused) { …………… …………. populate_rootfs();//检测initrd,initramdisk,cpio-initrd等 do_basic_setup(); /* * check if there is an early userspace init. If yes, let it do all * the work */ if (sys_access((const char __user *) "/init", 0) == 0)// execute_command = "/init"; else prepare_namespace();//挂载真正的根文件系统 //这之后真正的根文件系统已经建立 * Ok, we have completed the initial bootup, and * we're essentially up and running. Get rid of the * initmem segments and start the user-mode stuff.. */ free_initmem(); unlock_kernel(); system_state = SYSTEM_RUNNING; numa_default_policy(); if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0) printk(KERN_WARNING "Warning: unable to open an initial console./n"); (void) sys_dup(0); (void) sys_dup(0); /* * We try each of these until one succeeds. * * The Bourne shell can be used instead of init if we are * trying to recover a really broken machine. */ if (execute_command) run_init_process(execute_command); run_init_process("/sbin/init"); run_init_process("/etc/init"); run_init_process("/bin/init"); run_init_process("/bin/sh"); panic("No init found. Try passing init= option to kernel."); } 下面分析上面重要的函数 1. populate_rootfs负责检测initrd。 void __init populate_rootfs(void) { char *err = unpack_to_rootfs(__initramfs_start, __initramfs_end - __initramfs_start, 0); if (err) panic(err); #ifdef CONFIG_BLK_DEV_INITRD if (initrd_start) {//initrd检测 int fd; printk(KERN_INFO "checking if image is initramfs..."); err = unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start, 1); if (!err) { printk(" it is/n"); unpack_to_rootfs((char *)initrd_start, initrd_end - initrd_start, 0); free_initrd_mem(initrd_start, initrd_end); return; } printk("it isn't (%s); looks like an initrd/n", err); fd = sys_open("/initrd.image", O_WRONLY|O_CREAT, 700); if (fd >= 0) { sys_write(fd, (char *)initrd_start, initrd_end - initrd_start); //将内存中的initrd(通常由bootload加载到内存中)赋值到initrd.image中,以释放其占用的内存资源。 sys_close(fd); free_initrd_mem(initrd_start, initrd_end);//释放内存资源 } } #endif } 2.然后就是 prepare_namespace(),它负责具体根文件系统挂载。 void __init prepare_namespace(void) { int is_floppy; mount_devfs();//挂载devfs文件系统到/dev目录。这个是必须的,因为initrd要放到/dev/ram0里 if (root_delay) { printk(KERN_INFO "Waiting %dsec before mounting root device.../n", root_delay); ssleep(root_delay); } md_run_setup(); if (saved_root_name[0]) { root_device_name = saved_root_name; ROOT_DEV = name_to_dev_t(root_device_name); if (strncmp(root_device_name, "/dev/", 5) == 0) root_device_name += 5; } is_floppy = MAJOR(ROOT_DEV) == FLOPPY_MAJOR; if (initrd_load()) goto out; if (is_floppy && rd_doload && rd_load_disk(0)) ROOT_DEV = Root_RAM0;// //如果我们在bootoption哟参数root=/dev/mtdblock2,ROOT_DEV就为/dev/mtdblock设备号。 mount_root(); out: umount_devfs("/dev"); //devfs从虚拟的根文件系统的/dev umount sys_mount(".", "/", NULL, MS_MOVE, NULL);//将挂载点从当前目录 【/root】(在mount_root函数中设置的)移到根目录 sys_chroot("."); //将当前目录即【/root】(真正文件系统挂载的目录)做为系统根目录, security_sb_post_mountroot(); mount_devfs_fs ();//将devfs挂到真正根文件系统的/dev } void __init mount_root(void) { #ifdef CONFIG_ROOT_NFS if (MAJOR(ROOT_DEV) == UNNAMED_MAJOR) { if (mount_nfs_root()) return; printk(KERN_ERR "VFS: Unable to mount root fs via NFS, trying floppy./n"); ROOT_DEV = Root_FD0; } #endif #ifdef CONFIG_BLK_DEV_FD if (MAJOR(ROOT_DEV) == FLOPPY_MAJOR) { /* rd_doload is 2 for a dual initrd/ramload setup */ if (rd_doload==2) { if (rd_load_disk(1)) { ROOT_DEV = Root_RAM1; root_device_name = NULL; } } else change_floppy("root floppy"); } #endif create_dev("/dev/root", ROOT_DEV, root_device_name);//创建ROOT_DEV设备文件即为根文件系统设备文件。 mount_block_root("/dev/root", root_mountflags);//挂载根文件系统,root_mountflags为文件系统类型。 } void __init mount_block_root(char *name, int flags) { char *fs_names = __getname(); char *p; char b[BDEVNAME_SIZE]; get_fs_names(fs_names);//获得文件系统类型,如果在bootoption里有,则就为这个文件系统类型,如果没有指定,则返回filesytem链上所有类型,下面再对每个进行尝试. retry: for (p = fs_names; *p; p += strlen(p)+1) { int err = do_mount_root(name, p, flags, root_mount_data);//将文件系统挂到/root目录,p为文件系统类型,由get_fs_names得到 switch (err) { case 0: goto out; case -EACCES: flags |= MS_RDONLY; goto retry; case -EINVAL: continue; } /* * Allow the user to distinguish between failed sys_open * and bad superblock on root device. */ __bdevname(ROOT_DEV, b); printk("VFS: Cannot open root device /"%s/" or %s/n", root_device_name, b); printk("Please append a correct /"root=/" boot option/n"); panic("VFS: Unable to mount root fs on %s", b); } panic("VFS: Unable to mount root fs on %s", __bdevname(ROOT_DEV, b)); out: putname(fs_names); } static int __init do_mount_root(char *name, char *fs, int flags, void *data) { int err = sys_mount(name, "/root", fs, flags, data); if (err) return err; sys_chdir("/root");/将当前目录设为/root目录 ROOT_DEV = current->fs->pwdmnt->mnt_sb->s_dev; printk("VFS: Mounted root (%s filesystem)%s./n", current->fs->pwdmnt->mnt_sb->s_type->name, current->fs->pwdmnt->mnt_sb->s_flags & MS_RDONLY ? " readonly" : ""); return 0; } 到此根文件系统挂载完成。 整个函数调用路径如下 Start_kernel->rest_init->init-> prepare_namespace-> mount_root-> mount_block_root do_mount_root-> sys_mount(name, "/root", fs, flags, data)-> sys_chroot(".");

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