Ext3 mount過程分析
Ext3 mount原理本質上,Ext3 mount的過程實際上是inode被替代的過程。例如,/dev/sdb塊設備被mount到/mnt/alan目錄。那麼mount這個過程所需要解決的問題就是將/mnt/alan的dentry目錄項所指向的inode屏蔽掉,然後重新定位到/dev/sdb所表示的inode索引節點。在沒有分析閱讀linux vfs mount代碼的時候,我的想法是修改dentry所指向的inode索引節點,以此實現mount文件系統的訪問。經過分析,在實際的vfs mount實現過程中,還是和我原始的想法略有差別,但是,基本目標還是相同的。
Linux VFS的mount過程基本原理如下圖所示:
http://img1.51cto.com/attachment/201301/200930594.jpg
當用戶輸入”mount /dev/sdb /mnt/alan”命令後,Linux會解析/mnt/alan字符串,並且從Dentry Hash表中獲取相關的dentry目錄項,然後將該目錄項標識成DCACHE_MOUNTED。一旦該dentry被標識成DCACHE_MOUNTED,也就意味著在訪問路徑上對其進行了屏蔽。
在mount /dev/sdb設備上的ext3文件系統時,內核會創建一個該文件系統的superblock對象,並且從/dev/sdb設備上讀取所有的superblock信息,初始化該內存對象。Linux內核維護了一個全局superblock對象鍊錶。s_root是superblock對象所維護的dentry目錄項,該目錄項是該文件系統的根目錄。即新mount的文件系統內容都需要通過該根目錄進行訪問。在mount的過程中,VFS會創建一個非常重要的vfsmount對象,該對象維護了文件系統mount的所有信息。Vfsmount對象通過HASH表進行維護,通過path地址計算HASH值,在這裡vfsmount的HASH值通過“/mnt/alan”路徑字符串進行計算得到。Vfsmount中的mnt_root指向superblock對象的s_root根目錄項。因此,通過/mnt/alan地址可以檢索VFSMOUNT Hash Table得到被mount的vfsmount對象,進而得到mnt_root根目錄項。
例如,/dev/sdb被mount之後,用戶想要訪問該設備上的一個文件ab.c,假設該文件的地址為:/mnt/alan/ab.c。在打開該文件的時候,首先需要進行path解析。在解析到/mnt/alan的時候,得到/mnt/alan的dentry目錄項,並且發現該目錄項已經被標識為DCACHE_MOUNTED。之後,會採用/mnt/alan計算HASH值去檢索VFSMOUNT Hash Table,得到對應的vfsmount對象,然後採用vfsmount指向的mnt_root目錄項替代/mnt/alan原來的dentry,從而實現了dentry和inode的重定向。在新的dentry的基礎上,解析程序繼續執行,最終得到表示ab.c文件的inode對象。
關鍵數據結構說明
Linux VFS mount所涉及的關鍵數據結構分析如下。
Vfsmount數據結構
Vfsmount數據結構是vfs mount最為重要的數據結構,其維護了一個mount點的所有信息。該數據結構描述如下:
struct vfsmount {
struct list_head mnt_hash; /* 連接到VFSMOUNT Hash Table */
struct vfsmount *mnt_parent; /* 指向mount樹中的父節點*/
struct dentry *mnt_mountpoint; /* 指向mount點的目錄項*/
struct dentry *mnt_root; /* 被mount的文件系統根目錄項*/
struct super_block *mnt_sb; /* 指向被mount的文件系統superblock */
#ifdef CONFIG_SMP
struct mnt_pcp __percpu *mnt_pcp;
atomic_t mnt_longterm; /* how many of the refs are longterm */
#else
int mnt_count;
int mnt_writers;
#endif
struct list_head mnt_mounts; /* 下級(child)vfsmount對象鍊錶*/
struct list_head mnt_child; /* 鏈入上級vfsmount對象的鍊錶點*/
int mnt_flags;
/* 4 bytes hole on 64bits arches without fsnotify */
#ifdef CONFIG_FSNOTIFY
__u32 mnt_fsnotify_mask;
struct hlist_head mnt_fsnotify_marks;
#endif
const char *mnt_devname; /* 文件系統所在的設備名字,例如/dev/sdb */
struct list_head mnt_list;
struct list_head mnt_expire; /* link in fs-specific expiry list */
struct list_head mnt_share; /* circular list of shared mounts */
struct list_head mnt_slave_list;/* list of slave mounts */
struct list_head mnt_slave; /* slave list entry */
struct vfsmount *mnt_master; /* slave is on master- > mnt_slave_list */
struct mnt_namespace *mnt_ns; /* containing namespace */
int mnt_id; /* mount identifier */
int mnt_group_id; /* peer group identifier */
int mnt_expiry_mark; /* true if marked for expiry */
int mnt_pinned;
int mnt_ghosts;
};
在Linux內核中不僅存在VFSMOUNT的Hash Table,而且還維護了一棵Mount對象樹,通過該mount樹,我們可以了解到各個文件系統之間的關係。該mount樹描述如下:
http://img1.51cto.com/attachment/201301/202219326.jpg上圖所示為三層mount文件系統樹。第一層為系統根目錄“/”;第二層有兩個mount點,一個為/mnt/a,另一個是/mnt/b;第三層在/mnt/a的基礎上又創建了兩個mount點,分別為/mnt/a/c和/mnt/a/d。通過mount樹,可以對整個系統的mount結構一目了然。
Superblock數據結構
每個文件系統都會擁有一個superblock對像對其基本信息進行描述。對於像ext3之類的文件系統而言,在磁盤上會持久化存儲一份superblock元數據信息,內存的superblock對象由磁盤上的信息初始化。對於像block device 之類的“偽文件系統”而言,在mount的時候也會創建superblock對象,只不過很多信息都是臨時生成的,沒有持久化信息。Vfs superblock數據結構定義如下:
struct super_block {
struct list_head s_list; /* 鏈入全局鍊錶的對象*/
dev_t s_dev; /* search index; _not_ kdev_t */
unsigned char s_dirt;
unsigned char s_blocksize_bits;
unsigned long s_blocksize;
loff_t s_maxbytes; /* Max file size */
struct file_system_type *s_type;
const struct super_operations *s_op; /* superblock操作函數集*/
const struct dquot_operations *dq_op;
const struct quotactl_ops *s_qcop;
const struct export_operations *s_export_op;
unsigned long s_flags;
unsigned long s_magic;
struct dentry *s_root; /* 文件系統根目錄項*/
struct rw_semaphore s_umount;
struct mutex s_lock;
int s_count;
atomic_t s_active;
#ifdef CONFIG_SECURITY
void *s_security;
#endif
const struct xattr_handler **s_xattr;
struct list_head s_inodes; /* all inodes */
struct hlist_bl_head s_anon; /* anonymous dentries for (nfs) exporting */
#ifdef CONFIG_SMP
struct list_head __percpu *s_files;
#else
struct list_head s_files;
#endif
/* s_dentry_lru, s_nr_dentry_unused protected by dcache.c lru locks */
struct list_head s_dentry_lru; /* unused dentry lru */
int s_nr_dentry_unused; /* # of dentry on lru */
/* s_inode_lru_lock protects s_inode_lru and s_nr_inodes_unused */
spinlock_t s_inode_lru_lock ____cacheline_aligned_in_smp;
struct list_head s_inode_lru; /* unused inode lru */
int s_nr_inodes_unused; /* # of inodes on lru */
struct block_device *s_bdev;
struct backing_dev_info *s_bdi;
struct mtd_info *s_mtd;
struct list_head s_instances;
struct quota_info s_dquot; /* Diskquota specific options */
int s_frozen;
wait_queue_head_t s_wait_unfrozen;
char s_id; /* Informational name */
u8 s_uuid; /* UUID */
void *s_fs_info; /* Filesystem private info */
fmode_t s_mode;
/* Granularity of c/m/atime in ns.
Cannot be worse than a second */
u32 s_time_gran;
/*
* The next field is for VFS *only*. No filesystems have any business
* even looking at it. You had been warned.
*/
struct mutex s_vfs_rename_mutex; /* Kludge */
/*
* Filesystem subtype. If non-empty the filesystem type field
* in /proc/mounts will be "type.subtype"
*/
char *s_subtype;
/*
* Saved mount options for lazy filesystems using
* generic_show_options()
*/
char __rcu *s_options;
const struct dentry_operations *s_d_op; /* default d_op for dentries */
/*
* Saved pool identifier for cleancache (-1 means none)
*/
int cleancache_poolid;
struct shrinker s_shrink; /* per-sb shrinker handle */
};
代碼流程分析
Linux中實現mount操作需要一定的代碼量,下面對Linux VFS Mount代碼進行分析說明,整個分析過程按照mount操作函數調用流程進行。代碼分析基於Linux-3.2版本。
當用戶在用戶層執行mount命令時,會執行系統調用從用戶態陷入linux內核,執行如下函數(namespace.c):
SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
char __user *, type, unsigned long, flags, void __user *, data)
{
int ret;
char *kernel_type;
char *kernel_dir;
char *kernel_dev;
unsigned long data_page;
/* 獲取mount類型 */
ret=copy_mount_string (type, &kernel_type);
if (ret<0 )
goto out_type;
/* 獲取mount點目錄字符串 */
kernel_dir=getname (dir_name);
if (IS_ERR(kernel_dir)) {
ret=PTR_ERR (kernel_dir);
goto out_dir;
}
/* 獲取設備名稱字符串 */
ret=copy_mount_string (dev_name, &kernel_dev);
if (ret<0 )
goto out_dev;
/* 獲取其它選項 */
ret=copy_mount_options (data, &data_page);
if (ret<0 )
goto out_data;
/* 主要函數,執行掛載文件系統的具體操作*/
ret=do_mount (kernel_dev, kernel_dir, kernel_type, flags,
(void *) data_page);
free_page(data_page);
out_data:
kfree(kernel_dev);
out_dev:
putname(kernel_dir);
out_dir:
kfree(kernel_type);
out_type:
return ret;
}
do_mount()函數是mount操作過程中的核心函數,在該函數中,通過mount的目錄字符串找到對應的dentry目錄項,然後通過do_new_mount()函數完成具體的mount操作。do_mount()函數分析如下:
long do_mount(char *dev_name, char *dir_name, char *type_page,
unsigned long flags, void *data_page)
{
struct path path;
intretval=0 ;
intmnt_flags=0 ;
。。。
/* 通過mount目錄字符串獲取path,path結構中包含有mount目錄的dentry目錄對象*/
retval=kern_path (dir_name, LOOKUP_FOLLOW, &path);
if (retval)
return retval;
。。。
/* Separate the per-mountpoint flags */
if (flags & MS_NOSUID)
mnt_flags |= MNT_NOSUID;
if (flags & MS_NODEV)
mnt_flags |= MNT_NODEV;
if (flags & MS_NOEXEC)
mnt_flags |= MNT_NOEXEC;
if (flags & MS_NOATIME)
mnt_flags |= MNT_NOATIME;
if (flags & MS_NODIRATIME)
mnt_flags |= MNT_NODIRATIME;
if (flags & MS_STRICTATIME)
mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
if (flags & MS_RDONLY)
mnt_flags |= MNT_READONLY;
flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
MS_STRICTATIME);
/* remount操作 */
if (flags & MS_REMOUNT)
retval=do_remount (&path, flags & ~MS_REMOUNT, mnt_flags,
data_page);
else if (flags & MS_BIND)
retval=do_loopback (&path, dev_name, flags & MS_REC);
else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
retval=do_change_type (&path, flags);
else if (flags & MS_MOVE)
retval=do_move_mount (&path, dev_name);
else
/* 正常的mount操作,完成具體的mount操作*/
retval=do_new_mount (&path, type_page, flags, mnt_flags,
dev_name, data_page);
dput_out:
path_put(&path);
return retval;
}
do_new_mount()函數主要分成兩大部分:第一部分建立vfsmount對象和superblock對象,必要時從設備上獲取文件系統元數據;第二部分將vfsmount對象加入到mount樹和Hash Table中,並且將原來的dentry對象無效掉。do_new_mount函數說明如下:
static int do_new_mount(struct path *path, char *type, int flags,
int mnt_flags, char *name, void *data)
{
struct vfsmount *mnt;
int err;
。。。
/* 在內核建立vfsmount對象和superblock對象*/
mnt=do_kern_mount (type, flags, name, data);
if (IS_ERR(mnt))
return PTR_ERR(mnt);
/* 將vfsmount對象加入系統,屏蔽原有dentry對象*/
err=do_add_mount (mnt, path, mnt_flags);
if (err)
mntput(mnt);
return err;
}
do_new_mount()中的第一步調用do_kern_mount()函數,該函數的主幹調用路徑如下:do_kern_mount--> vfs_kern_mount--> mount_fs
在mount_fs()函數中會調用特定文件系統的mount方法,如果mount是ext3文件系統,那麼在mount_fs函數中最終會調用ext3的mount方法。Ext3的mount方法定義在super.c文件中:
static struct file_system_typeext3_fs_type= {
.owner =THIS_MODULE ,
.name ="ext3" ,
.mount =ext3_mount , /* ext3文件系統mount方法*/
.kill_sb =kill_block_super ,
.fs_flags =FS_REQUIRES_DEV ,
};
Ext3 mount函數主幹調用路徑為:ext3_mount--> mount_bdev。Mount_bdev()函數主要完成superblock對象的內存初始化,並且加入到全局superblock鍊錶中。該函數說明如下:
struct dentry *mount_bdev(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data,
int (*fill_super)(struct super_block *, void *, int))
{
struct block_device *bdev;
struct super_block *s;
fmode_tmode=FMODE_READ| FMODE_EXCL;
interror=0 ;
if (!(flags & MS_RDONLY))
mode |= FMODE_WRITE;
/* 通過設備名字獲取被mount設備的bdev對象*/
bdev=blkdev_get_by_path (dev_name, mode, fs_type);
if (IS_ERR(bdev))
return ERR_CAST(bdev);
/*
* once the super is inserted into the list by sget, s_umount
* will protect the lockfs code from trying to start a snapshot
* while we are mounting
*/
mutex_lock(&bdev- > bd_fsfreeze_mutex);
if (bdev- > bd_fsfreeze_count>0) {
mutex_unlock(&bdev- > bd_fsfreeze_mutex);
error= -EBUSY;
goto error_bdev;
}
/* 查找或者創建superblock對象 */
s=sget (fs_type, test_bdev_super, set_bdev_super, bdev);
mutex_unlock(&bdev- > bd_fsfreeze_mutex);
if (IS_ERR(s))
goto error_s;
if (s- > s_root) {
/* 被mount文件系統的根目錄項已經存在*/
if ((flags ^ s- > s_flags) & MS_RDONLY) {
deactivate_locked_super(s);
error= -EBUSY;
goto error_bdev;
}
/*
* s_umount nests inside bd_mutex during
* __invalidate_device(). blkdev_put() acquires
* bd_mutex and can't be called under s_umount. Drop
* s_umount temporarily. This is safe as we're
* holding an active reference.
*/
up_write(&s- > s_umount);
blkdev_put(bdev, mode);
down_write(&s- > s_umount);
} else {
/* 文件系統根目錄項不存在,通過filler_super函數讀取磁盤上的superblock元數據信息,並且初始化superblock內存結構*/
char b;
s- > s_flags= flags | MS_NOSEC;
s- > s_mode= mode;
strlcpy(s- > s_id, bdevname(bdev, b), sizeof(s- > s_id));
sb_set_blocksize(s, block_size(bdev));
/* 對於ext3文件系統,調用ext3_fill_super函數*/
error=fill_super (s, data, flags & MS_SILENT ? 1 : 0);
if (error) {
deactivate_locked_super(s);
goto error;
}
s- > s_flags |= MS_ACTIVE;
bdev- > bd_super= s;
}
/* 正常返回被mount文件系統根目錄項*/
return dget(s- > s_root);
error_s:
error=PTR_ERR (s);
error_bdev:
blkdev_put(bdev, mode);
error:
return ERR_PTR(error);
}
do_new_mount()函數的第二步是將創建的vfsmount對象加入到mount樹和VFSMOUNT Hash Table中,並且將老的dentry目錄項無效掉。該過程主幹函數調用過程如下所示:do_new_mount--> do_add_mount--> graft_tree--> attach_recursive_mnt
attach_recursive_mnt()函數完成第二步過程的主要操作。至此,文件系統的mount操作已經完成。Mount完成之後,如果用戶想要訪問新mount文件系統中的文件,那麼需要在path解析過程中重定位dentry,該過程主要在follow_managed()函數中完成。在該函數中會判斷一個dentry是否已經被標識成DCACHE_MOUNTED,如果該標誌位已經被設置,那麼通過VFSMOUNT Hash Table可以重定位dentry。
如有不對之處,敬請指出更正(tl_wzj@yahoo.com.cn )。
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