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

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Message-ID: <20250814171941.GU7942@frogsfrogsfrogs>
Date: Thu, 14 Aug 2025 10:19:41 -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 03:58:58PM +0200, Miklos Szeredi wrote:
> On Thu, 3 Jul 2025 at 20:54, John Groves <John@...ves.net> wrote:
> >
> > * The new GET_DAXDEV message/response is enabled
> > * The command it triggered by the update_daxdev_table() call, if there
> >   are any daxdevs in the subject fmap that are not represented in the
> >   daxdev_dable yet.
> 
> This is rather convoluted, the server *should know* which dax devices
> it has registered, hence it shouldn't need to be explicitly asked.
> 
> And there's already an API for registering file descriptors:
> FUSE_DEV_IOC_BACKING_OPEN.  Is there a reason that interface couldn't
> be used by famfs?

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?

--D

> Thanks,
> Miklos
>