Hope support overlayfs upper layer on juicefs

[davies]

Discussed in #2114

^{Originally posted by wuyongzhi May 10, 2022}

when i create overlay fs with juicefs as upper layer, i can not create overlay fs, it got a error:

the command:

# mount -t overlay o1 -o lowerdir=jfs/exercises/001,upperdir=jfs/user/001,workdir=workspaces/001/work workspaces/001/merged

output:

mount: /opt/workspaces/001/merged: wrong fs type, bad option, bad superblock on o1, missing codepage or helper program, or other error

What would you like to be added:
create overlay fs with juicefs as upper layer,

Why is this needed:

[SandyXSD]

Generally FUSE and network filesystems are not allowed as upper layer for overlayfs, see: https://github.com/torvalds/linux/blob/v5.4/fs/overlayfs/super.c#L754-L759 and: https://github.com/torvalds/linux/blob/v5.4/fs/overlayfs/util.c#L105.
FUSE dentry has DCACHE_OP_REVALIDATE flag set as it implements d_revalidate:
https://github.com/torvalds/linux/blob/v5.4/fs/fuse/dir.c#L301