Re: [PATCH] Fuse: Add backing file support for uring_cmd

[Miklos Szeredi <miklos@szeredi.hu>]

On Fri, 21 Feb 2025 at 19:31, Amir Goldstein <amir73il@gmail.com> wrote:

> BTW, I am now trying to work out the API for setting up a backing file
> for an inode at LOOKUP time for passthrough of inode operations.
> For this mode of operation, I was considering to support OPEN
> response with FOPEN_PASSTHROUGH and zero backing_id to mean
> "the backing file that is associated with the inode".
> I've actually reserved backing_id 0 for this purpose.
> In this mode of operations the problem at hand will become moot.
>
> One way to deal with the API of FOPEN_PASSTHROUGH in
> io_uring is to only use this mode of operation.
> IOW, LOOKUP response could have a backing fd and not
> a backing id and then the backing ids are not even exposed to
> server because the server does not care - for all practical purposes
> the nodeid is the backing id.

Yeah, the backing-id thing should not be needed for io-uring.

One complaint about the current passthrough API is that it adds extra
syscalls, which is expensive nowadays.

> I personally don't mind if inode operations passthrough
> that are setup via LOOKUP response, will require io_uring.
> Both features are about metadata operations performance,
> so it kind of makes sense to bundle them together, does it not?

Right, this would be the least complex solution.   We could also add
an ioctl(FUSE_DEV_IOC_LOOKUP_REPLY), which would work with the
non-uring API.

Thanks,
Miklos