linux-kernel - Re: [PATCH 06/67] nfs, cifs, ceph, 9p: Disable use of fscache prior to its rewrite

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Date:   Wed, 20 Oct 2021 11:57:43 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Dave Wysochanski <dwysocha@...hat.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        linux-nfs@...r.kernel.org, Ilya Dryomov <idryomov@...il.com>,
        ceph-devel@...r.kernel.org, Steve French <sfrench@...ba.org>,
        linux-cifs@...r.kernel.org, Eric Van Hensbergen <ericvh@...il.com>,
        Latchesar Ionkov <lucho@...kov.net>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        v9fs-developer@...ts.sourceforge.net,
        Trond Myklebust <trondmy@...merspace.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/67] nfs, cifs, ceph, 9p: Disable use of fscache prior to its rewrite

Jeff Layton <jlayton@...nel.org> wrote:

> The typical way to do this would be to rebrand the existing FSCACHE
> Kconfig symbols into FSCACHE_OLD and then build the new fscache
> structure such that it exists in parallel with the old.

That, there, is nub of the problem.

You can't have parallel cachefiles drivers: There's a single userspace
interface (/dev/cachefiles) and only one driver can register it.  You would
need to decide at compile time whether you want the converted or the
unconverted network filesystems to be cached.

> You'd then just drop the old infrastructure once all of the fs's are
> converted to the new. You could even make them conflict with one another in
> Kconfig too, so that only one could be built in during the transition period
> if supporting both at runtime is too difficult.
> 
> This approach of disabling everything is much more of an all-or-nothing
> affair. It may mean less "churn" overall, but it seems less "nice"
> because you have an interval of commits where fscache is non-functional.
> 
> I'm not necessarily opposed to this approach, but I'd like to better
> understand why doing it this way was preferred.

I'm trying to avoid adding two parallel drivers, but change in place so that I
can test parts of it as I go along.

David