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

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Message-ID: <20250814182022.GW7942@frogsfrogsfrogs>
Date: Thu, 14 Aug 2025 11:20:22 -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 12/18] famfs_fuse: Plumb the GET_FMAP message/response

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.

>  - 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. :)

>  - 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.

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

More or less.

> 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.

--D

> Thanks,
> Miklos
> 
> >
> > Thanks,
> > Miklos
>