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

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Message-ID: <mwnzafopjjpcgf3mznua3nphgmhdpiaato5pvojz7uz3bdw57n@zl7x2uz2hkfj>
Date: Fri, 15 Aug 2025 11:53:16 -0500
From: John Groves <John@...ves.net>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: 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>, 
	"Darrick J . Wong" <djwong@...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>, john@...ves.net
Subject: Re: [RFC V2 12/18] famfs_fuse: Plumb the GET_FMAP message/response

On 25/08/14 04:36PM, 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
>  - allow several types of backing files: say regular and dax dev
>  - querying maps has a common protocol, format of maps is opaque to this
>  - maps are cached by a common facility
>  - each type of mapping has a decoder module
>  - each type of backing file has a module for handling I/O
> 
> Does this make sense?
> 
> 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.

Maybe GET_FILE_MAP or GET_FILE_IOMAP if we want to keep overloading 
the term iomap. Maps are to backing-dev for regular file systems,
and to device memory (devdax) for famfs - in all cases both read
and write (when write is allowed).

Thanks,
John