Re: [PATCH 1/3] Implement generic freeze feature

Andrew Morton
2008-08-21T19:58:23Z

On Mon, 18 Aug 2008 21:28:19 +0900
Takashi Sato <t-sato@yk.jp.nec.com> wrote:

> 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
>
>
> ...
>
> --- linux-2.6.27-rc2.org/include/linux/fs.h 2008-08-06 13:49:54.000000000 +0900
> +++ linux-2.6.27-rc2-freeze/include/linux/fs.h 2008-08-07 08:59:54.000000000 +0900
> @@ -226,6 +226,8 @@ extern int dir_notify_enable;
> #define BMAP_IOCTL 1 /* obsolete - kept for compatibility */
> #define FIBMAP _IO(0x00,1) /* bmap access */
> #define FIGETBSZ _IO(0x00,2) /* get the block size used for bmap */
> +#define FIFREEZE _IOWR('X', 119, int) /* Freeze */
> +#define FITHAW _IOWR('X', 120, int) /* Thaw */

FIFREEZE is 119, but a few lines above we have

#define BLKDISCARD _IO(0x12,119)

Should we be using 120 and 121 here?

> #define FS_IOC_GETFLAGS _IOR('f', 1, long)
> #define FS_IOC_SETFLAGS _IOW('f', 2, long)
> @@ -574,6 +576,10 @@ struct block_device {
> * care to not mess up bd_private for that case.
> */
> unsigned long bd_private;
> + /* The counter of freeze processes */
> + int bd_freeze_count;
> + /* Semaphore for freeze */
> + struct semaphore bd_freeze_sem;

"freeze" is not an adequate description of what this protects. I think
it's only the modification and testing of bd_freeze_count, isn't it?

If so, all this could be done more neatly by removing the lock,
switching to atomic_t and using our (rich) atomic_t operations.

otoh, perhaps it protects more than this, in which case the lock
can/should be switched to a `struct mutex'?