filesystem freeze: implement generic freeze feature

The ioctls for the generic freeze feature are below.
o Freeze the filesystem
  int ioctl(int fd, int FIFREEZE, arg)
    fd: The file descriptor of the mountpoint
    FIFREEZE: request code for the freeze
    arg: Ignored
    Return value: 0 if the operation succeeds. Otherwise, -1

o Unfreeze the filesystem
  int ioctl(int fd, int FITHAW, arg)
    fd: The file descriptor of the mountpoint
    FITHAW: request code for unfreeze
    arg: Ignored
    Return value: 0 if the operation succeeds. Otherwise, -1
    Error number: If the filesystem has already been unfrozen,
                  errno is set to EINVAL.

[akpm@linux-foundation.org: fix CONFIG_BLOCK=n]
Signed-off-by: Takashi Sato <t-sato@yk.jp.nec.com>
Signed-off-by: Masayuki Hamaguchi <m-hamaguchi@ys.jp.nec.com>
Cc: <xfs-masters@oss.sgi.com>
Cc: <linux-ext4@vger.kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Kleikamp <shaggy@austin.ibm.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Alasdair G Kergon <agk@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 113 insertions, 9 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index ac7031f12ea5..b3c1efff5e1d 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -285,6 +285,8 @@ static void init_once(void *foo)
         INIT_LIST_HEAD(&bdev->bd_holder_list);
 #endif
         inode_init_once(&ei->vfs_inode);
+        /* Initialize mutex for freeze. */
+        mutex_init(&bdev->bd_fsfreeze_mutex);
 }
 
 static inline void __bd_forget(struct inode *inode)
diff --git a/fs/buffer.c b/fs/buffer.c
index 87f9e537b8c3..b6e8b8632e2f 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -203,10 +203,25 @@ int fsync_bdev(struct block_device *bdev)
  * happen on bdev until thaw_bdev() is called.
  * If a superblock is found on this device, we take the s_umount semaphore
  * on it to make sure nobody unmounts until the snapshot creation is done.
+ * The reference counter (bd_fsfreeze_count) guarantees that only the last
+ * unfreeze process can unfreeze the frozen filesystem actually when multiple
+ * freeze requests arrive simultaneously. It counts up in freeze_bdev() and
+ * count down in thaw_bdev(). When it becomes 0, thaw_bdev() will unfreeze
+ * actually.
  */
 struct super_block *freeze_bdev(struct block_device *bdev)
 {
         struct super_block *sb;
+        int error = 0;
+
+        mutex_lock(&bdev->bd_fsfreeze_mutex);
+        if (bdev->bd_fsfreeze_count > 0) {
+                bdev->bd_fsfreeze_count++;
+                sb = get_super(bdev);
+                mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                return sb;
+        }
+        bdev->bd_fsfreeze_count++;
 
         down(&bdev->bd_mount_sem);
         sb = get_super(bdev);
@@ -221,11 +236,24 @@ struct super_block *freeze_bdev(struct block_device *bdev)
 
                 sync_blockdev(sb->s_bdev);
 
-                if (sb->s_op->freeze_fs)
-                        sb->s_op->freeze_fs(sb);
+                if (sb->s_op->freeze_fs) {
+                        error = sb->s_op->freeze_fs(sb);
+                        if (error) {
+                                printk(KERN_ERR
+                                        "VFS:Filesystem freeze failed\n");
+                                sb->s_frozen = SB_UNFROZEN;
+                                drop_super(sb);
+                                up(&bdev->bd_mount_sem);
+                                bdev->bd_fsfreeze_count--;
+                                mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                                return ERR_PTR(error);
+                        }
+                }
         }
 
         sync_blockdev(bdev);
+        mutex_unlock(&bdev->bd_fsfreeze_mutex);
+
         return sb;      /* thaw_bdev releases s->s_umount and bd_mount_sem */
 }
 EXPORT_SYMBOL(freeze_bdev);
@@ -237,20 +265,48 @@ EXPORT_SYMBOL(freeze_bdev);
  *
  * Unlocks the filesystem and marks it writeable again after freeze_bdev().
  */
-void thaw_bdev(struct block_device *bdev, struct super_block *sb)
+int thaw_bdev(struct block_device *bdev, struct super_block *sb)
 {
+        int error = 0;
+
+        mutex_lock(&bdev->bd_fsfreeze_mutex);
+        if (!bdev->bd_fsfreeze_count) {
+                mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                return -EINVAL;
+        }
+
+        bdev->bd_fsfreeze_count--;
+        if (bdev->bd_fsfreeze_count > 0) {
+                if (sb)
+                        drop_super(sb);
+                mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                return 0;
+        }
+
         if (sb) {
                 BUG_ON(sb->s_bdev != bdev);
-
-                if (sb->s_op->unfreeze_fs)
-                        sb->s_op->unfreeze_fs(sb);
-                sb->s_frozen = SB_UNFROZEN;
-                smp_wmb();
-                wake_up(&sb->s_wait_unfrozen);
+                if (!(sb->s_flags & MS_RDONLY)) {
+                        if (sb->s_op->unfreeze_fs) {
+                                error = sb->s_op->unfreeze_fs(sb);
+                                if (error) {
+                                        printk(KERN_ERR
+                                                "VFS:Filesystem thaw failed\n");
+                                        sb->s_frozen = SB_FREEZE_TRANS;
+                                        bdev->bd_fsfreeze_count++;
+                                        mutex_unlock(&bdev->bd_fsfreeze_mutex);
+                                        return error;
+                                }
+                        }
+                        sb->s_frozen = SB_UNFROZEN;
+                        smp_wmb();
+                        wake_up(&sb->s_wait_unfrozen);
+                }
                 drop_super(sb);
         }
 
         up(&bdev->bd_mount_sem);
+        mutex_unlock(&bdev->bd_fsfreeze_mutex);
+        return 0;
 }
 EXPORT_SYMBOL(thaw_bdev);
 
diff --git a/fs/ioctl.c b/fs/ioctl.c
index cc3f1aa1cf7b..20b0a8a24c6b 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -439,6 +439,43 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp,
         return error;
 }
 
+static int ioctl_fsfreeze(struct file *filp)
+{
+        struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
+
+        if (!capable(CAP_SYS_ADMIN))
+                return -EPERM;
+
+        /* If filesystem doesn't support freeze feature, return. */
+        if (sb->s_op->freeze_fs == NULL)
+                return -EOPNOTSUPP;
+
+        /* If a blockdevice-backed filesystem isn't specified, return. */
+        if (sb->s_bdev == NULL)
+                return -EINVAL;
+
+        /* Freeze */
+        sb = freeze_bdev(sb->s_bdev);
+        if (IS_ERR(sb))
+                return PTR_ERR(sb);
+        return 0;
+}
+
+static int ioctl_fsthaw(struct file *filp)
+{
+        struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
+
+        if (!capable(CAP_SYS_ADMIN))
+                return -EPERM;
+
+        /* If a blockdevice-backed filesystem isn't specified, return EINVAL. */
+        if (sb->s_bdev == NULL)
+                return -EINVAL;
+
+        /* Thaw */
+        return thaw_bdev(sb->s_bdev, sb);
+}
+
 /*
  * When you add any new common ioctls to the switches above and below
  * please update compat_sys_ioctl() too.
@@ -486,6 +523,15 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
                 } else
                         error = -ENOTTY;
                 break;
+
+        case FIFREEZE:
+                error = ioctl_fsfreeze(filp);
+                break;
+
+        case FITHAW:
+                error = ioctl_fsthaw(filp);
+                break;
+
         default:
                 if (S_ISREG(filp->f_path.dentry->d_inode->i_mode))
                         error = file_ioctl(filp, cmd, arg);