linux-kernel - Re: [RFC PATCH 13/19] famfs_fuse: Create files with famfs fmaps

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Message-ID: <CAJfpegtR28rH1VA-442kS_ZCjbHf-WDD+w_FgrAkWDBxvzmN_g@mail.gmail.com>
Date: Tue, 6 May 2025 18:56:29 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: "Darrick J. Wong" <djwong@...nel.org>
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>, 
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	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
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	Jeff Layton <jlayton@...nel.org>, Kent Overstreet <kent.overstreet@...ux.dev>, 
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	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>, 
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	Josef Bacik <josef@...icpanda.com>, Aravind Ramesh <arramesh@...ron.com>, 
	Ajay Joshi <ajayjoshi@...ron.com>, 0@...ves.net
Subject: Re: [RFC PATCH 13/19] famfs_fuse: Create files with famfs fmaps

On Mon, 28 Apr 2025 at 21:00, Darrick J. Wong <djwong@...nel.org> wrote:

> <nod> I don't know what Miklos' opinion is about having multiple
> fusecmds that do similar things -- on the one hand keeping yours and my
> efforts separate explodes the amount of userspace abi that everyone must
> maintain, but on the other hand it then doesn't couple our projects
> together, which might be a good thing if it turns out that our domain
> models are /really/ actually quite different.

Sharing the interface at least would definitely be worthwhile, as
there does not seem to be a great deal of difference between the
generic one and the famfs specific one.  Only implementing part of the
functionality that the generic one provides would be fine.

> (Especially because I suspect that interleaving is the norm for memory,
> whereas we try to avoid that for disk filesystems.)

So interleaved extents are just like normal ones except they repeat,
right?  What about adding a special "repeat last N extent
descriptions" type of extent?

> > But the current implementation does not contemplate partially cached fmaps.
> >
> > Adding notification could address revoking them post-haste (is that why
> > you're thinking about notifications? And if not can you elaborate on what
> > you're after there?).
>
> Yeah, invalidating the mapping cache at random places.  If, say, you
> implement a clustered filesystem with iomap, the metadata server could
> inform the fuse server on the local node that a certain range of inode X
> has been written to, at which point you need to revoke any local leases,
> invalidate the pagecache, and invalidate the iomapping cache to force
> the client to requery the server.
>
> Or if your fuse server wants to implement its own weird operations (e.g.
> XFS EXCHANGE-RANGE) this would make that possible without needing to
> add a bunch of code to fs/fuse/ for the benefit of a single fuse driver.

Wouldn't existing invalidation framework be sufficient?

Thanks,
Miklos