linux-kernel - Re: inodes: Support generic defragmentation

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Date:	Mon, 8 Feb 2010 11:40:38 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Dave Chinner <david@...morbit.com>, tytso@....edu,
	Andi Kleen <andi@...stfloor.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Alexander Viro <viro@....linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Christoph Lameter <clameter@....com>,
	Rik van Riel <riel@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	akpm@...ux-foundation.org, Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org
Subject: Re: inodes: Support generic defragmentation

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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