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From: Rusty Russell <rusty@rustcorp.com.au>
To: Eric Dumazet <dada1@cosmosbay.com>
Subject: Re: [PATCH] modules: Use a better scheme for refcounting
Date: Fri, 16 May 2008 10:09:11 +1000
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Cc: Andrew Morton <akpm@linux-foundation.org>,
       linux kernel <linux-kernel@vger.kernel.org>,
       Mike Travis <travis@sgi.com>
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On Friday 16 May 2008 06:40:37 Eric Dumazet wrote:
> Current refcounting for modules (done if CONFIG_MODULE_UNLOAD=y)
> is using a lot of memory.

Hi Eric,

   I like this patch!  The plan was always to create a proper dynamic per-cpu
allocator which used the normal per-cpu offsets, but I think module refcounts
are worthwhile as a special case.

   Any chance I can ask you look at the issue of full dynamic per-cpu
allocation?  The problem of allocating memory which is laid out precisely
as the original per-cpu alloc is vexing on NUMA, and probably requires
reserving virtual address space and remapping into it, but the rewards
would be maximally-efficient per-cpu accessors, and getting rid of that
boutique allocator in module.c.

Only minor commentry on the patch itself:

> +#ifdef CONFIG_SMP
> +       char *refptr;
> +#else

void * would seem more natural here (love those gcc'isms)

> +#if defined(CONFIG_MODULE_UNLOAD) && defined(CONFIG_SMP)
> +       void *refptr = NULL;
> +#endif

Looks like you can skip this if you assign to mod->refptr directly below:

> +#if defined(CONFIG_MODULE_UNLOAD) && defined(CONFIG_SMP)
> +       refptr = percpu_modalloc(sizeof(local_t), sizeof(local_t), mod->name);
> +       if (!refptr)
> +               goto free_mod;
> +       mod->refptr = refptr;
> +#endif

And finally:

> +#if defined(CONFIG_MODULE_UNLOAD) && defined(CONFIG_SMP)
> +       if (refptr)
> +               percpu_modfree(refptr);
> +#endif

This if (refptr) seems redundant.

Thanks!
Rusty.