[CRIU] Dumping a FUSE mount

[Jonathan 'Wolf' Rentzsch]

> On Oct 1, 2015, at 4:23 AM,Pavel Emelyanov <xemul at parallels.com> wrote:
> Second option is to mark this thing with the --ext-mount-map flag thus
> telling criu to put only the mountpoint info into images on dump and pull
> them back on restore. The only problem here is that validate_mounts()
> only supports non-root mounts as external:
> 
>                 if (fsroot_mounted(m)) {
>                         if (m->fstype->code == FSTYPE__UNSUPPORTED) {
>                                 pr_err("FS mnt %s dev %#x root %s unsupported id %d\n",
>                                                 m->mountpoint, m->s_dev, m->root, m->mnt_id);
>                                 return -1;
>                         }
>                 } else if (!m->external) {
>                         ...
> 
> So the if branch will have to be patched to allow for external root mount.
> 
> >From my perspective the 2nd way is better.

Pavel, I went with your recommended second option.

I commented-out the validate_mounts() code as suggested, and was then able to dump. Yay.

However I’m not abel to successfully restore. do_mount_one() ends up calling do_new_mount(), which fails with 

1: Error (mount.c:1480): Can't mount at ./app/available_modules: No such device

I put in some logging and it’s calling

mount(“dupe”, "./app/available_modules”, “unsupported”, 0x200001, "user_id=0,group_id=0”)

“dupe" is the name of the external fuse process.

I tried detecting the “dupe” source argument and dynamically switching mount()’s filesystemtype arg from “unsupported” to “fuse”, as http://fuse.sourceforge.net/doxygen/ seems to suggest, but that just yields an "Invalid argument” error instead.

Any suggestions about how to get mount restoration working with an external fuse?

Note: We’re currently using criu version 1.5.2 in production.

-Wolf