linux-kernel - Re: [PATCH 10/28] FS-Cache: Recruit a couple of page flags for cache management [try #2]

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Date:	Thu, 20 Dec 2007 18:33:39 +0000
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	dhowells@...hat.com, viro@....linux.org.uk, hch@...radead.org,
	Trond.Myklebust@...app.com, sds@...ho.nsa.gov,
	casey@...aufler-ca.com, linux-kernel@...r.kernel.org,
	selinux@...ho.nsa.gov, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 10/28] FS-Cache: Recruit a couple of page flags for cache management [try #2]

Nick Piggin <nickpiggin@...oo.com.au> wrote:

> > > I'd much prefer if you would handle this in the filesystem, and have it
> > > set PG_private whenever fscache needs to receive a callback, and DTRT
> > > depending on whether PG_fscache etc. is set or not.
> >
> > That's tricky and slower[*].  One of the things I want to do is to modify
> > iso9660 to do be able to do caching, but PG_private is 'owned' by the
> > generic buffer cache code.
> 
> Maybe it is harder, but it is the right way to do it.

You're wrong.  It would mean that PG_private is the logical disjunction of
PG_fscache and some condition not otherwise explicitly stored.  I tried that
with NFS and it was nasty.

As you can no doubt see, it means that you can't distinguish all the states
you used to be able to.

> So you should modify the filesystems rather than core code.

I think you missed what I said:

	but PG_private is 'owned' by the generic buffer cache code.

That means more of the core code would have to change - or, at least, change
more.

David
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