Re: [PATCH] vfs: introduce FS_IOC_SYNCFS to sync a single super

[Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>]

On Mon, 9 Aug 2010 07:33:57 -0700 (PDT)
Sage Weil <sage-BnTBU8nroG7k1uMJSBkQmQ@public.gmane.org> wrote:

> Currently the only way to sync a single super_block (and not all of them 
> via sync(2)) is via the BLKFLSBUF ioctl on the block device.  That also 
> invalidates the bdev mapping, which isn't usually desireable

Actually you can do

	mount -o remount /dev/whatever

and it will sync the fs and retain caches.

> and it 
> doesn't work for non-block file systems.

And I guess remount will do that also.

>  The ability to sync a single 
> mount can be useful for both applications and administrators (e.g., when 
> other mounts on the system are hung).
> 
> Introduce a simple ioctl to sync the super associated with an open file.
> Pass any error returned by sync_filesystem() back to the user.
> 

The changelog forgot to tell us why this is a useful thing to add. 
What is the use-case?

> ---
>  fs/ioctl.c         |    9 +++++++++
>  include/linux/fs.h |    1 +
>  2 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 2d140a7..2aabb19 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -593,6 +593,15 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd,
>  	case FS_IOC_FIEMAP:
>  		return ioctl_fiemap(filp, arg);
>  
> +	case FS_IOC_SYNCFS:
> +	{
> +		struct super_block *sb = filp->f_dentry->d_sb;
> +		down_read(&sb->s_umount);
> +		error = sync_filesystem(sb);
> +		up_read(&sb->s_umount);
> +		break;
> +	}
> +

`mount -o remount' is surely a Linux-specific side-effect and there's
really no guarantee that Linux will always retain that side-effect. 
OTOH FS_IOC_SYNCFS is linux-specific.

If we're going to add something like this then it will need to be
documented in manpages.  Supposedly, a cc to linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
will help make all that happen, but I'm not sure who if anyone is
answering the phone over there?