linux-kernel - Re: [PATCH 02/14] FS-Cache: Recruit a couple of page flags for cache management

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Message-ID: <17631.1185956833@redhat.com>
Date:	Wed, 01 Aug 2007 09:27:13 +0100
From:	David Howells <dhowells@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	torvalds@...l.org, akpm@...l.org, steved@...hat.com,
	trond.myklebust@....uio.no, linux-fsdevel@...r.kernel.org,
	linux-cachefs@...hat.com, nfsv4@...ux-nfs.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/14] FS-Cache: Recruit a couple of page flags for cache management

Peter Zijlstra <peterz@...radead.org> wrote:

> Not sure its a good idea to overload page_has_private() with an
> overloadable page-flag. What if some future FS wants to use
> PG_owner_priv_2 for other purposes?

All that it means is that releasepage() and co will get called if a page is to
be released or invalidated that has that bit set.  I think that's something a
future FS could probably live with.

However, I do have to trigger a call to releasepage() and co *somehow*.

> Obviously filesystems cannot use these two page-flags if they want to be
> compatible with FS-cache, but need all filesystems be?

What do you mean?  That's why I went for the PG_owner_priv_* approach rather
than just naming the bits unto FS-Cache directly.

> (also, ouch! - 2 pageflags)

Yeah.  The consequence of having things asynchronous is that you have to find
signalling mechanisms to synchronise around the asynchronicity:-/

Furthermore, it occurs to me that I can't use PG_private or page->private to
store this information because I want to make isofs use caching, and those two
pieces of information are owned by the buffering code.

David
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