Re: [Qemu-devel] RFC: moving fsfreeze support from the userland guest agent to the guest kernel

[Fernando Luis Vázquez Cao]

On Wed, 2011-07-27 at 17:24 +0200, Andrea Arcangeli wrote:
> making
> sure no lib is calling any I/O function to be able to defreeze the
> filesystems later, making sure the oom killer or a wrong kill -9
> $RANDOM isn't killing the agent by mistake while the I/O is blocked
> and the copy is going.

Yes with the current API if the agent is killed while the filesystems
are frozen we are screwed.

I have just submitted patches that implement a new API that should make
the virtualization use case more reliable. Basically, I am adding a new
ioctl, FIGETFREEZEFD, which freezes the indicated filesystem and returns
a file descriptor; as long as that file descriptor is held open, the
filesystem remains open. If the freeze file descriptor is closed (be it
through a explicit call to close(2) or as part of process exit
housekeeping) the associated filesystem is automatically thawed.

- fsfreeze: add ioctl to create a fd for freeze control
  http://marc.info/?l=linux-fsdevel&m=131175212512290&w=2
- fsfreeze: add freeze fd ioctls
  http://marc.info/?l=linux-fsdevel&m=131175220612341&w=2