Re: [PATCH v3 3/4] limit nr_dentries per superblock

From: Dave Chinner
Date: Mon Aug 15 2011 - 06:47:05 EST

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@xxxxxxxxxxxxx> 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....


Dave Chinner
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