[PATCH] ubifs: Add freeze support

[Christoph Hellwig]

On Fri, May 26, 2017 at 11:52:42AM +0200, Richard Weinberger wrote:
> Hyunchul,
> 
> Am 26.05.2017 um 01:30 schrieb Hyunchul Lee:
> > From: Hyunchul Lee <cheol.lee at lge.com>
> > 
> > for un/freeze support, implement freeze_super and un/freeze_fs
> > of super_operations.
> > ubifs_freeze_super just calls freeze_super. because freeze_super always
> > succeeds if file system is read-only,  UBIFS errors should be checked.
> > if there are errors, UBIFS is switched to read-only mode.
> > ubifs_freeze_fs runs commit if TNC/LPT isn't clean. though all writes
> > are blocked and sync_fs is called before, if commit alreay was started
> > before writes are blocked, TNC/LPT might have dirty COW nodes.
> 
> you explain how you implement that feature, but not why.
> What is the use-case?
> I always thought this interface is only being used by LVM.

It can also be used through the FIFREEZE ioctl, but without snapshots
underneath the fs it's not all that useful.