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From: Kent Overstreet <kent.overstreet@linux.dev>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Christoph Hellwig <hch@infradead.org>, ksummit@lists.linux.dev,
	linux-fsdevel@vger.kernel.org
Subject: Re: [MAINTAINERS/KERNEL SUMMIT] Trust and maintenance of file systems
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On Sat, Sep 09, 2023 at 08:50:39AM -0400, James Bottomley wrote:
> So why can't we figure out that easier way? What's wrong with trying to
> figure out if we can do some sort of helper or library set that assists
> supporting and porting older filesystems. If we can do that it will not
> only make the job of an old fs maintainer a lot easier, but it might
> just provide the stepping stones we need to encourage more people climb
> up into the modern VFS world.

What if we could run our existing filesystem code in userspace?

bcachefs has a shim layer (like xfs, but more extensive) to run nearly
the entire filesystem - about 90% by loc - in userspace.

Right now this is used for e.g. userspace fsck, but one of my goals is
to have the entire filesystem available as a FUSE filesystem. I'd been
planning on doing the fuse port as a straight fuse implementation, but
OTOH if we attempted a sh vfs iops/aops/etc. -> fuse shim, then we would
have pretty much everything we need to run any existing fs (e.g.
reiserfs) as a fuse filesystem.

It'd be a nontrivial project with some open questions (e.g. do we have
to lift all of bufferheads to userspace?) but it seems worth
investigating.