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

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Date:	Fri, 21 Dec 2007 12:08:23 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Howells <dhowells@...hat.com>
Cc:	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]

On Friday 21 December 2007 05:33, David Howells wrote:
> 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.

Then make a PG_private2 bit and use that.

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