Date: Mon, 8 Feb 2010 11:40:38 -0600 (CST)
From: Christoph Lameter <cl@linux-foundation.org>
To: Nick Piggin <npiggin@suse.de>
cc: Dave Chinner <david@fromorbit.com>, tytso@mit.edu,
       Andi Kleen <andi@firstfloor.org>, Miklos Szeredi <miklos@szeredi.hu>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Christoph Hellwig <hch@infradead.org>,
       Christoph Lameter <clameter@sgi.com>, Rik van Riel <riel@redhat.com>,
       Pekka Enberg <penberg@cs.helsinki.fi>, akpm@linux-foundation.org,
       Nick Piggin <nickpiggin@yahoo.com.au>, Hugh Dickins <hugh@veritas.com>,
       linux-kernel@vger.kernel.org
Subject: Re: inodes: Support generic defragmentation
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On Mon, 8 Feb 2010, Nick Piggin wrote:

> > > After that, LRU on slabs should be fairly easy. Slab could provide a
> > > private per-slab pointer for example that is managed by the caller.
> > > Subsystem can then call into slab to find the objects.
> >
> > Sure with some minor changes we could have a call that is giving you the
> > list of neighboring objects in a slab, while locking it? Then you can look
> > at the objects and decide which ones can be tossed and then do another
> > call to release the objects and unlock the slab.
>
> Yep. Well... you may not even need to ask slab layer to lock the
> slab. Provided that the subsystem is locking out changes. It could
> possibly be helpful to have a call to lock and unlock the slab,
> although usage of such an API would have to be very careful.

True, if you are holding a reference to an object in a slab page and
there is a guarantee that the object is not going away then the slab is already
effectively pinned.

So we just need a call that returns

1. The number of allocated objects in a slab page
2. The total possible number of objects
3. A list of pointers to the objects

?

Then reclaim could make a decision if you want these objects to be
reclaimed.

Such a function could actually be a much less code than the current
patchset and would also be easy to do for SLAB/SLOB.


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