Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system

[Miklos Szeredi <miklos@szeredi.hu>]

On Thu, 29 Feb 2024 at 07:52, Amir Goldstein <amir73il@gmail.com> wrote:

> I'm not virtiofs expert, but I don't think that you are wrong about this.
> IIUC, virtiofsd could map arbitrary memory region to any fuse file mmaped
> by virtiofs client.
>
> So what are the gaps between virtiofs and famfs that justify a new filesystem
> driver and new userspace API?

Let me try to fill in some gaps.  I've looked at the famfs driver
(even tried to set it up in a VM, but got stuck with the EFI stuff).

- famfs has an extent list per file that indicates how each page
within the file should be mapped onto the dax device, IOW it has the
following mapping:

  [famfs file, offset] -> [offset, length]

- fuse can currently map a fuse file onto a backing file:

  [fuse file] -> [backing file]

The interface for the latter is

   backing_id = ioctl(dev_fuse_fd, FUSE_DEV_IOC_BACKING_OPEN, backing_map);
...
   fuse_open_out.flags |= FOPEN_PASSTHROUGH;
   fuse_open_out.backing_id = backing_id;

This looks suitable for doing the famfs file - > dax device mapping as
well.  I wouldn't extend the ioctl with extent information, since
famfs can just use FUSE_DEV_IOC_BACKING_OPEN once to register the dax
device.  The flags field could be used to tell the kernel to treat
this fd as a dax device instead of a a regular file.

Letter, when the file is opened the extent list could be sent in the
open reply together with the backing id.  The fuse_ext_header
mechanism seems suitable for this.

And I think that's it as far as API's are concerned.

Note: this is already more generic than the current famfs prototype,
since multiple dax devices could be used as backing for famfs files,
with the constraint that a single file can only map data from a single
dax device.

As for implementing dax passthrough, I think that needs a separate
source file, the one used by virtiofs (fs/fuse/dax.c) does not appear
to have many commonalities with this one.  That could be renamed to
virtiofs_dax.c as it's pretty much virtiofs specific, AFAICT.

Comments?  Am I missing something significant?

Thanks,
Miklos