linux-kernel - Re: [PATCH v3 3/4] limit nr

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Date:	Mon, 15 Aug 2011 20:46:56 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Glauber Costa <glommer@...allels.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	James Bottomley <JBottomley@...allels.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Christoph Lameter <cl@...ux.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3 3/4] limit nr_dentries per superblock

On Mon, Aug 15, 2011 at 10:12:06AM +0300, Pekka Enberg wrote:
> On Sun, Aug 14, 2011 at 6:13 PM, Glauber Costa <glommer@...allels.com> wrote:
> > This patch lays the foundation for us to limit the dcache size.
> > Each super block can have only a maximum amount of dentries under its
> > sub-tree. Allocation fails if we we're over limit and the cache
> > can't be pruned to free up space for the newcomers.
.....
> We track the total number of objects in mm/slub.c when
> CONFIG_SLUB_DEBUG is enabled (look for n->total_objects in the code).
> Have you considered extending that for this purpose?

That's usage for the entire slab, though, and we don't have a dentry
slab per superblock so I don't think that helps us. And with slab
merging, I think that even if we did have a slab per superblock,
they'd end up in the same slab context anyway, right?

Ideally what we need is a slab, LRU and shrinkers all rolled into a
single infrastructure handle so we can simply set them up per
object, per context etc and not have to re-invent the wheel for
every single slab cache/LRU/shrinker setup we have in the kernel.

I've got a rough node-aware generic LRU/shrinker infrastructure
prototype that is generic enough for most of the existing slab
caches with shrinkers, but I haven't looked at what is needed to
integrate it with the slab cache code. That's mainly because I don't
like the idea of having to implement the same thing 3 times in 3
different ways and debug them all before anyone would consider it
for inclusion in the kernel.

Once I've sorted out the select_parent() use-the-LRU-for-disposal
abuse and have a patch set that survives a 'rm -rf *' operation,
maybe we can then talk about what is needed to integrate stuff into
the slab caches....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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