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Message-ID: <CAJfpegv8XzFvty_x00UehUQxw9ai8BytvGNXE8SL03zfsTN6ag@mail.gmail.com>
Date: Fri, 17 May 2024 11:55:38 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Amir Goldstein <amir73il@...il.com>
Cc: John Groves <John@...ves.net>, John Groves <jgroves@...ron.com>,
Jonathan Corbet <corbet@....net>, Dan Williams <dan.j.williams@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>,
Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Matthew Wilcox <willy@...radead.org>, linux-cxl@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev, john@...alactic.com,
Dave Chinner <david@...morbit.com>, Christoph Hellwig <hch@...radead.org>, dave.hansen@...ux.intel.com,
gregory.price@...verge.com, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system
On Thu, 29 Feb 2024 at 07:52, Amir Goldstein <amir73il@...il.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
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