linux-kernel - Re: [RFC PATCH 00/19] famfs: port into fuse

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Message-ID: <n4idslcksqlegd5e2nyggyzahwhgfh7qqsh3qni7fgpj342ohr@do2fu5pz7fsv>
Date: Wed, 30 Apr 2025 19:13:37 -0700
From: John Groves <John@...ves.net>
To: Alireza Sanaee <alireza.sanaee@...wei.com>
Cc: Dan Williams <dan.j.williams@...el.com>, 
	Miklos Szeredi <miklos@...redb.hu>, 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>, Luis Henriques <luis@...lia.com>, 
	Randy Dunlap <rdunlap@...radead.org>, Jeff Layton <jlayton@...nel.org>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, Petr Vorel <pvorel@...e.cz>, Brian Foster <bfoster@...hat.com>, 
	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 PATCH 00/19] famfs: port into fuse

On 25/04/30 03:42PM, Alireza Sanaee wrote:
> On Sun, 20 Apr 2025 20:33:27 -0500
> John Groves <John@...ves.net> wrote:
> 
>> <snip>
> 
> Hi John,
> 
> Apologies if the question is far off or irrelevant.
> 
> I am trying to understand FAMFS, and I am thinking where does FAMFS
> stand when compared to OpenSHMEM PGAS. Can't we have a OpenSHMEM-based
> shared memory implementation over CXL that serves as FAMFS?
> 
> Maybe FAMFS does more than that!?!
> 
> Thanks,
> Alireza
>

Continuation of this conversation likely belongs in the discusison section
at [1], but a couple of thoughts.

Famfs provides a scale-out filesystem mounts where the files that map to the
same disaggregated shared memory. If you mmap a famfs file, you are accessing
the memory directly. Since shmem is file-backed (usually tmpfs or
its ilk), shmem is a higher-level and more specialized abstraction, and
OpenSHMEM may be able to run atop famfs. It looks like OpenSHMEM and PGAS
cover the possibility that "shared memory" might require grabbing a copy via
[r]dma - which famfs will probably never do. Famfs only handles cases where
the memory is actually shared. (hey, I work for a memory company.)

Since famfs provides memory-mappable files, almost all apps can access them
(no requirement to write to the shmem, or other related but more estoteric
interfaces). Apps are responsible for not doing "nonsense" access WRT cache
coherency, but famfs manages cache coherency for its metadata.

The video at [2] may be useful to get up to speed.

[1] http://github.com/cxl-micron-reskit/famfs
[2] https://www.youtube.com/watch?v=L1QNpb-8VgM&t=1680