From: Dan Magenheimer <dan.magenheimer@oracle.com>
Subject: RE: [PATCH V3 0/8] Cleancache: overview
Date: Fri, 23 Jul 2010 10:37:51 -0700 (PDT)
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Cc: akpm@linux-foundation.org, Chris Mason <chris.mason@oracle.com>,
        viro@zeniv.linux.org.uk, adilger@sun.com, tytso@mit.edu,
        mfasheh@suse.com, Joel Becker <joel.becker@oracle.com>, matthew@wil.cx,
        linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
        ocfs2-devel@oss.oracle.com, linux-mm@kvack.org, jeremy@goop.org,
        JBeulich@novell.com, Kurt Hackel <kurt.hackel@oracle.com>,
        npiggin@suse.de, Dave Mccracken <dave.mccracken@oracle.com>,
        riel@redhat.com, avi@redhat.com, Konrad Wilk <konrad.wilk@oracle.com>
To: Christoph Hellwig <hch@infradead.org>, ngupta@vflare.org
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> From: Dan Magenheimer
> Subject: RE: [PATCH V3 0/8] Cleancache: overview
>=20
> > From: Christoph Hellwig [mailto:hch@infradead.org]
> > Subject: Re: [PATCH V3 0/8] Cleancache: overview
> >
> > On Fri, Jul 23, 2010 at 06:58:03AM -0700, Dan Magenheimer wrote:
> > > CHRISTOPH AND ANDREW, if you disagree and your concerns have
> > > not been resolved, please speak up.
>=20
> Hi Christoph --
>=20
> Thanks very much for the quick (instantaneous?) reply!
>=20
> > Anything that need modification of a normal non-shared fs is utterly
> > broken and you'll get a clear NAK, so the propsal before is a good
> > one.
>=20
> Unless/until all filesystems are 100% built on top of VFS,
> I have to disagree.  Abstractions (e.g. VFS) are never perfect.

After thinking about this some more, I can see a way
to enforce "opt-in" in the cleancache backend without
any changes to non-generic fs code.   I think it's a horrible
hack and we can try it, but I expect fs maintainers
would prefer the explicit one-line-patch opt-in.

1) Cleancache backend maintains a list of "known working"
   filesystems (those that have been tested).

2) Nitin's proposed changes pass the *sb as a parameter.
  The string name of the filesystem type is available via
  sb->s_type->name.  This can be compared against
  the "known working" list.

Using the sb pointer as a "handle" requires an extra
table search on every cleancache get/put/flush,
and fs/super.c changes are required for fs unmount
notification anyway (e.g. to call cleancache_flush_fs)
so I'd prefer to keep the cleancache_poolid addition
to the sb.  I'll assume this is OK since this is in generic
fs code.

Dan

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