Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 8 Mar 2021 13:54:10 -0500
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Amir Goldstein <amir73il@gmail.com>, linux-cachefs@redhat.com,
        Jeff Layton <jlayton@redhat.com>,
        David Wysochanski <dwysocha@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <dchinner@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-afs@lists.infradead.org,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        CIFS <linux-cifs@vger.kernel.org>,
        ceph-devel <ceph-devel@vger.kernel.org>,
        v9fs-developer@lists.sourceforge.net,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: fscache: Redesigning the on-disk cache
Message-ID: <20210308185410.GE7284@fieldses.org>
References: <CAOQ4uxhxwKHLT559f8v5aFTheKgPUndzGufg0E58rkEqa9oQ3Q@mail.gmail.com>
 <2653261.1614813611@warthog.procyon.org.uk>
 <517184.1615194835@warthog.procyon.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <517184.1615194835@warthog.procyon.org.uk>
User-Agent: Mutt/1.5.21 (2010-09-15)
Precedence: bulk

On Mon, Mar 08, 2021 at 09:13:55AM +0000, David Howells wrote:
> Amir Goldstein <amir73il@gmail.com> wrote:
> > With ->fiemap() you can at least make the distinction between a non existing
> > and an UNWRITTEN extent.
> 
> I can't use that for XFS, Ext4 or btrfs, I suspect.  Christoph and Dave's
> assertion is that the cache can't rely on the backing filesystem's metadata
> because these can arbitrarily insert or remove blocks of zeros to bridge or
> split extents.

Could you instead make some sort of explicit contract with the
filesystem?  Maybe you'd flag it at mkfs time and query for it before
allowing a filesystem to be used for fscache.  You don't need every
filesystem to support fscache, right, just one acceptable one?

--b.