Re: [RFC PATCH 0/9] fuse: API for Checkpoint/Restore

[Bernd Schubert]

On 2/20/23 20:37, Alexander Mikhalitsyn wrote:
> Hello everyone,
>
> It would be great to hear your comments regarding this proof-of-concept Checkpoint/Restore API for FUSE.
>
> Support of FUSE C/R is a challenging task for CRIU [1]. Last year I've given a brief talk on LPC 2022
> about how we handle files C/R in CRIU and which blockers we have for FUSE filesystems. [2]
>
> The main problem for CRIU is that we have to restore mount namespaces and memory mappings before the process tree.
> It means that when CRIU is performing mount of fuse filesystem it can't use the original FUSE daemon from the
> restorable process tree, but instead use a "fake daemon".
>
> This leads to many other technical problems:
> * "fake" daemon has to reply to FUSE_INIT request from the kernel and initialize fuse connection somehow.
> This setup can be not consistent with the original daemon (protocol version, daemon capabilities/settings
> like no_open, no_flush, readahead, and so on).
> * each fuse request has a unique ID. It could confuse userspace if this unique ID sequence was reset.
>
> We can workaround some issues and implement fragile and limited support of FUSE in CRIU but it doesn't make any sense, IMHO.
> Btw, I've enumerated only CRIU restore-stage problems there. The dump stage is another story...
>
> My proposal is not only about CRIU. The same interface can be useful for FUSE mounts recovery after daemon crashes.
> LXC project uses LXCFS [3] as a procfs/cgroupfs/sysfs emulation layer for containers. We are using a scheme when
> one LXCFS daemon handles all the work for all the containers and we use bindmounts to overmount particular
> files/directories in procfs/cgroupfs/sysfs. If this single daemon crashes for some reason we are in trouble,
> because we have to restart all the containers (fuse bindmounts become invalid after the crash).
> The solution is fairly easy:
> allow somehow to reinitialize the existing fuse connection and replace the daemon on the fly
> This case is a little bit simpler than CRIU cause we don't need to care about the previously opened files
> and other stuff, we are only interested in mounts.


I like your patches, small and easy to read :)
So this basically fails all existing open files - our (future) needs go 
beyond that. I wonder if we can extend it later and re-init the new 
daemon with something like "fuse_queue_recall" - basically the opposite 
of fuse_queue_forget. Not sure if fuse can access the vfs dentry cache 
to know for which files that would need to be done - if not, it would 
need to do its own book-keeping.


Thanks,
Bernd