freeze feature ver 1.13

Review

- Rename write_super_lockfs/unlockfs of the super block operation freeze_fs/unfreeze_fs to avoid a confusion.

Currently, ext3 in mainline Linux doesn't have the freeze feature which suspends write requests. So, we cannot take a backup which keeps the filesystem's consistency with the storage device's features (snapshot and replication) while it is mounted.
In many case, a commercial filesystem (e.g. VxFS) has
the freeze feature and it would be used to get the consistent backup. If Linux's standard filesystem ext3 has the freeze feature, we can do it without a commercial filesystem.

So I have implemented the ioctls of the freeze feature. I think we can take the consistent backup with the following steps.
1. Freeze the filesystem with the freeze ioctl.
2. Separate the replication volume or create the snapshot with the storage device's feature.
3. Unfreeze the filesystem with the unfreeze ioctl.
4. Take the backup from the separated replication volume or the snapshot.

[PATCH 1/10] VFS: Fix error handling of write_super_lockfs/unlockfs
Changed the type of write_super_lockfs and unlockfs from "void" to "int" so that they can return an error.
Rename write_super_lockfs/unlockfs of the super block operation freeze_fs/unfreeze_fs to avoid a confusion.

[PATCH 2/10]-[PATCH 6/10] Fix error handling in write_super_lockfs/unlockfs

                          (ext3, ext4, xfs, gfs2, jfs)

Changed write_super_lockfs so that it returns an error if needed. unlockfs always returns 0.

[PATCH 7/10] reiserfs: Fix error handling in write_super_lockfs/unlockfs
Changed write_super_lockfs/unlockfs so that they always return 0 (success) to keep a current behavior.

[PATCH 8/10] 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

[PATCH 9/10] Remove XFS specific ioctl interfaces for freeze feature
It removes XFS specific ioctl interfaces and request codes for freeze feature.
This patch has been supplied by David Chinner.

[PATCH 10/10] Add timeout feature
The timeout feature is added to "freeze ioctl" to solve a deadlock when the freezer accesses a frozen filesystem. And new ioctl to reset the timeout period is added to extend the timeout period. For example, the freezer resets the timeout period to 10 seconds every 5 seconds. In this approach, even if the freezer causes a deadlock by accessing the frozen filesystem, it will be solved by the timeout in 10 seconds and the freezer will be able to recognize that at the next reset of timeout period.
o Freeze the filesystem
int ioctl(int fd, int FIFREEZE, long *timeout_sec)
fd: The file descriptor of the mountpoint
FIFREEZE: request code for the freeze
timeout_sec: the timeout period in seconds
If it's 0 or 1, the timeout isn't set.
This special case of "1" is implemented to keep the compatibility with XFS applications.
Return value: 0 if the operation succeeds. Otherwise, -1

o Reset the timeout period
This is useful for the application to set the timeout_sec more accurately. For example, the freezer resets the timeout_sec to 10 seconds every 5 seconds. In this approach, even if the freezer causes a deadlock by accessing the frozen filesystem, it will be solved by the timeout in 10 seconds and the freezer can recognize that at the next reset of timeout_sec.
int ioctl(int fd, int FIFREEZE_RESET_TIMEOUT, long *timeout_sec)
fd:file descriptor of mountpoint
FIFREEZE_RESET_TIMEOUT: request code for reset of timeout period
timeout_sec: new timeout period in seconds
Return value: 0 if the operation succeeds. Otherwise, -1 Error number: If the filesystem has already been unfrozen,
errno is set to EINVAL.

Any comments are very welcome.

Cheers, Takashi