linux-kernel - Re: [RFC V2 14/18] famfs_fuse: GET_DAXDEV message and daxdev

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Message-ID: <20250814185503.GZ7942@frogsfrogsfrogs>
Date: Thu, 14 Aug 2025 11:55:03 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: John Groves <John@...ves.net>, Dan Williams <dan.j.williams@...el.com>,
	Bernd Schubert <bschubert@....com>,
	John Groves <jgroves@...ron.com>, Jonathan Corbet <corbet@....net>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Jeff Layton <jlayton@...nel.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Amir Goldstein <amir73il@...il.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Stefan Hajnoczi <shajnocz@...hat.com>,
	Joanne Koong <joannelkoong@...il.com>,
	Josef Bacik <josef@...icpanda.com>,
	Aravind Ramesh <arramesh@...ron.com>,
	Ajay Joshi <ajayjoshi@...ron.com>
Subject: Re: [RFC V2 14/18] famfs_fuse: GET_DAXDEV message and daxdev_table

On Thu, Aug 14, 2025 at 08:25:23PM +0200, Miklos Szeredi wrote:
> On Thu, 14 Aug 2025 at 19:19, Darrick J. Wong <djwong@...nel.org> wrote:
> > What happens if you want to have a fuse server that hosts both famfs
> > files /and/ backing files?  That'd be pretty crazy to mix both paths in
> > one filesystem, but it's in theory possible, particularly if the famfs
> > server wanted to export a pseudofile where everyone could find that
> > shadow file?
> 
> Either FUSE_DEV_IOC_BACKING_OPEN detects what kind of object it has
> been handed, or we add a flag that explicitly says this is a dax dev
> or a block dev or a regular file.  I'd prefer the latter.

I don't think it's difficult to do something like:

	if (!fud)
		return -EPERM;

	if (copy_from_user(&map, argp, sizeof(map)))
		return -EFAULT;

	if (IS_ENABLED(CONFIG_FUSE_IOMAP)) {
		ret = fuse_iomap_dev_add(fud->fc, &map);
		if (ret)
			return ret;
	}

	if (IS_ENABLED(CONFIG_FUSE_PASSTHROUGH))
		return fuse_backing_open(fud->fc, &map);

	return 0;

I guess the hard part is -- how do we return /two/ device cookies?
Or do we move the backing_files_map out of CONFIG_FUSE_PASSTHROUGH and
then let fuse-iomap/famfs extract the block/dax device from that?
Then the backing_id/device cookie would be the same across a fuse mount.
iomap would have to check that it's being given block devices, but
that's easy.

--D

> Thanks,
> Miklos
>