[CRIU] [RFC] Fuse "stub-proxy" file system and how it can help to migrate network file system

[Stanislav Kinsburskiy]

Hello,

There is a class of objects, which CRIU can't migrate. They are network 
file systems and all the related objects like opened and mapped files.
The major problem with migration of a network file system is that it can 
be unreachable when restore is happening, while all opened and mapped 
files have to be restored somehow.

Another file system can become a solution for this problem.
Below are main characteristics of it:
1) Fuse-based filesystem to make it generic for any kernel.
2) Some methods: open() and mmap() (probably getattr() as well) return 
some dummy handle. This is required to be able to restore and remap 
files on restore.
3) Other methods are stubs: any process, called such a method will be 
put to sleep, waiting for original network file system is remounted.
4) Once it's remounted, this files system starts working as proxy, 
bypassing requests to desired file system.

This approach allows to restore a container with any network file system 
inside by mounting fuse fs instead of unreachable network file system..
But this proxy mode is slow.

Fortunately, in can be optimized.
CRIU can wait till real file system is remounted, and after that seize 
all the processes, using fuse, reopen (or map) a real file and replace 
fuse's files.
This will allow to restore native state of the processes and proxy file 
system can be shut down.

But still one more problem is left. Real file system have to be mounted 
to the same dentry, where fuse fs is mounted. There is no atomic way of 
replacing one file system with another. Unmounting fuse first and 
mounting real fs afterwards gives a race window, which is unacceptable. 
Thus only mounting of the real file system on top of fuse in suitable. 
But in this case fuse can be unmounted. Thus, alien mount point appears 
after restore.

This problem can be solved by freezing all the process to eliminate the 
race window mentioned above by the following algorithm:
1) Mounting real file system to some other dentry first.
2) Freezing all processes.
3) Lazy unmounting fuse.
4) Moving network file system to the desired dentry
5) Thawing all processes.

With this solution fuse mount point will disappear once all file 
references are closed.