Re: virtiofs uuid and file handles

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, Sep 23, 2020 at 4:49 AM Amir Goldstein <amir73il@gmail.com> wrote:

> I think that the proper was to implement reliable persistent file
> handles in fuse/virtiofs would be to add ENCODE/DECODE to
> FUSE protocol and allow the server to handle this.

Max Reitz (Cc-d) is currently looking into this.

One proposal was to add  LOOKUP_HANDLE operation that is similar to
LOOKUP except it takes a {variable length handle, name} as input and
returns a variable length handle *and* a u64 node_id that can be used
normally for all other operations.

The advantage of such a scheme for virtio-fs (and possibly other fuse
based fs) would be that userspace need not keep a refcounted object
around until the kernel sends a FORGET, but can prune its node ID
based cache at any time.   If that happens and a request from the
client (kernel) comes in with a stale node ID, the server will return
-ESTALE and the client can ask for a new node ID with a special
lookup_handle(fh, NULL).

Disadvantages being:

 - cost of generating a file handle on all lookups
 - cost of storing file handle in kernel icache

I don't think either of those are problematic in the virtiofs case.
The cost of having to keep fds open while the client has them in its
cache is much higher.

Thanks,
Miklos