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SubjectRe: [PATCH] vfs: introduce FS_IOC_SYNCFS to sync a single super
On Mon, 9 Aug 2010 07:33:57 -0700 (PDT)
Sage Weil <sage@newdream.net> 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@vger.kernel.org
will help make all that happen, but I'm not sure who if anyone is
answering the phone over there?



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