linux-kernel - Re: [PATCH 06/43] FS-Cache: Recruit a couple of page flags for cache management [ver #46]

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Date:	Thu, 02 Apr 2009 15:36:12 +0100
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	dhowells@...hat.com, viro@...iv.linux.org.uk, nfsv4@...ux-nfs.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 06/43] FS-Cache: Recruit a couple of page flags for cache management [ver #46]

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

> 1) PG_mappedtodisk is basically PG_owner_priv_2. Please alias that and
> use it? Then at least we're down to 1 extra flag.
> 2) Why do you need another PG_private?  PG_private for pagecache means
> that it should call into the filesystem when it needs to handle fs data
> attached to the page, right? So PG_private_2 doesn't really make sense
> in that respect.

Won't that either break fs/buffer.c and fs/mpage.c or preclude the use of
FS-Cache with block-based filesystems that use the standard buffer wangling
routines?

As I've previously stated, I want to be able to make ISO9660 use FS-Cache.
That rules out use of PG_mappedtodisk and PG_private for anything FS-Cache
related.

We can actually reclaim PG_private, I think.  There are patches to do that.
At the very least, we can probably reclaim the std buffering code's use of it.

If anything, avoiding the need for PG_fscache_write is probably easier - just
more memory intensive and slower.  I could build a second radix tree for each
inode that kept track of which pages from that inode FS-Cache knows about, and
use the status bits in that node to keep track of what pages are being written
out to the cache.

We still need a way of triggering the page invalidation callbacks for in-use
pages, however.  PG_private, as I've said, is not currently a viable option.

David
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