linux-kernel - Re: [RFC V2 12/18] famfs_fuse: Plumb the GET

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Message-ID: <hd3tancdc6pgjka44nwhk6laawnasob44jqagwxawrmxtevihe@2orrcse6xyjx>
Date: Fri, 15 Aug 2025 10:06:01 -0500
From: John Groves <John@...ves.net>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Miklos Szeredi <miklos@...redi.hu>, 
	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 12/18] famfs_fuse: Plumb the GET_FMAP message/response

On 25/08/14 11:20AM, Darrick J. Wong wrote:
> On Thu, Aug 14, 2025 at 04:36:57PM +0200, Miklos Szeredi wrote:
> > On Thu, 14 Aug 2025 at 15:36, Miklos Szeredi <miklos@...redi.hu> wrote:
> > 
> > > I'm still hoping some common ground would benefit both interfaces.
> > > Just not sure what it should be.
> > 
> > Something very high level:
> > 
> >  - allow several map formats: say a plain one with a list of extents
> > and a famfs one
> 
> Yes, I think that's needed.

Agreed

> 
> >  - allow several types of backing files: say regular and dax dev
> 
> "block device", for iomap.
> 
> >  - querying maps has a common protocol, format of maps is opaque to this
> >  - maps are cached by a common facility
> 
> I've written such a cache already. :)

I guess I need to take a look at that. Can you point me to the right place?

> 
> >  - each type of mapping has a decoder module
> 
> I don't know that you need much "decoding" -- for famfs, the regular
> mappings correspond to FUSE_IOMAP_TYPE_MAPPED.  The one goofy part is
> the device cookie in each IO mapping: fuse-iomap maps each block device
> you give it to a device cookie, so I guess famfs will have to do the
> same.
> 
> OTOH you can then have a famfs backed by many persistent memory
> devices.

That's handled in the famfs fmaps already. When an fmap is ingested,
if it references any previously-unknown daxdevs, they get retrieved
(FUSE_GET_DAXDEV).

Oversimplifying a bit, I assume that famfs fmaps won't really change,
they'll just be retrieved by a more flexible method and be preceded
by a header that identifies the payload as a famfs fmap.

> 
> >  - each type of backing file has a module for handling I/O
> > 
> > Does this make sense?
> 
> More or less.

I'm nervous about going for too much generalization too soon here,
but otherwise yeah.

> 
> > This doesn't have to be implemented in one go, but for example
> > GET_FMAP could be renamed to GET_READ_MAP with an added offset and
> > size parameter.  For famfs the offset/size would be set to zero/inf.
> > I'd be content with that for now.
> 
> I'll try to cough up a RFC v4 next week.

Darrick, let's try to chat next week to compare notes.

Based on this thinking, I will keep my rework of GET_FMAP to a minimum
since that will likely be a new shared message/response. I think that
part can be merged later in the cycle...

John