linux-ext4 - RE: [PATCH V3 0/8] Cleancache: overview

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Date:	Fri, 23 Jul 2010 10:37:51 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>, ngupta@...are.org
Cc:	akpm@...ux-foundation.org, Chris Mason <chris.mason@...cle.com>,
	viro@...iv.linux.org.uk, adilger@....com, tytso@....edu,
	mfasheh@...e.com, Joel Becker <joel.becker@...cle.com>,
	matthew@....cx, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, ocfs2-devel@....oracle.com,
	linux-mm@...ck.org, jeremy@...p.org, JBeulich@...ell.com,
	Kurt Hackel <kurt.hackel@...cle.com>, npiggin@...e.de,
	Dave Mccracken <dave.mccracken@...cle.com>, riel@...hat.com,
	avi@...hat.com, Konrad Wilk <konrad.wilk@...cle.com>
Subject: RE: [PATCH V3 0/8] Cleancache: overview

> From: Dan Magenheimer
> Subject: RE: [PATCH V3 0/8] Cleancache: overview
> 
> > From: Christoph Hellwig [mailto:hch@...radead.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.
> 
> Hi Christoph --
> 
> Thanks very much for the quick (instantaneous?) reply!
> 
> > 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.
> 
> 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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