linux-kernel - Re: fscache: Redesigning the on-disk cache

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Message-ID: <20210308185410.GE7284@fieldses.org>
Date:   Mon, 8 Mar 2021 13:54:10 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Amir Goldstein <amir73il@...il.com>, linux-cachefs@...hat.com,
        Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-afs@...ts.infradead.org,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: fscache: Redesigning the on-disk cache

On Mon, Mar 08, 2021 at 09:13:55AM +0000, David Howells wrote:
> Amir Goldstein <amir73il@...il.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.