linux-kernel - Re: [RFC V2 11/18] famfs_fuse: Basic famfs mount opts

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Message-ID: <z56yzi6y4hbbvcwpqzysbmztdhgsuqavjbnhsjxp3iumzvvywv@ymudodg3mb5x>
Date: Thu, 14 Aug 2025 09:39:07 -0500
From: John Groves <John@...ves.net>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: "Darrick J. Wong" <djwong@...nel.org>, 
	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 11/18] famfs_fuse: Basic famfs mount opts

On 25/08/14 12:37PM, Miklos Szeredi wrote:
> On Sat, 12 Jul 2025 at 07:54, Darrick J. Wong <djwong@...nel.org> wrote:
> >
> > On Fri, Jul 11, 2025 at 10:28:20AM -0500, John Groves wrote:
> 
> > >     famfs_fuse: Basic famfs mount opt: -o shadow=<shadowpath>
> > >
> > >     The shadow path is a (usually tmpfs) file system area used by the famfs
> > >     user space to commuicate with the famfs fuse server. There is a minor
> > >     dilemma that the user space tools must be able to resolve from a mount
> > >     point path to a shadow path. The shadow path is exposed via /proc/mounts,
> > >     but otherwise not used by the kernel. User space gets the shadow path
> > >     from /proc/mounts...
> 
> Don't know if we want to go that way.  Is there no other way?
> 
> But if we do, at least do it in a generic way.  I.e. fuse server can
> tell the kernel to display options A, B and C in /proc/mounts.
> 
> Thanks,
> Miklos

So far I haven't come up with an alternative, other than bad ones. 

Could parse the shadow path from the fuse server with the correct mount
point from 'ps -ef', but there are cases where a fuse server is killed and 
the kernel still thinks it's mounted (and we still might need to find the 
shadow path).

Could write the shadow path to a systemd log and parse it from there, but 
that would break if the log wasn't enabled, and would disappear if the log
was rotated during a long-running mount - and this resolution must happen
every time the famfs cli does most anything (cp, creat, fsck, etc.).

Could write it to a "secret file" somewhere, but that's kinda brittle.

Shadow paths are almost always tmpdir paths that are generated at mount time,
so there really isn't a good way to guess them, and it doesn't seem viable
to require them to be in (e.g.) /tmp in all cases.

Here is what it currently looks like on a running system:

$ grep famfs /proc/mounts
/dev/dax0.0 /mnt/famfs fuse rw,nosuid,nodev,relatime,user_id=0,group_id=0,shadow=/tmp/famfs_shadow_5m0dnH 0 0
$ ps -ef | grep /mnt/famfs | grep -v grep
root       12775       1  0 07:04 ?        00:00:00 /dev/dax0.0 -o daxdev=/dev/dax0.0,shadow=/tmp/famfs_shadow_5m0dnH,fsname=/dev/dax0.0,timeout=31536000.000000 /mnt/famfs

Having a generic approach rather than a '-o' option would be fine with me.
Also happy to entertain other ideas...

Thanks,
John