linux-kernel - Re: [PATCH] modules: Use a better scheme for refcounting

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Date:	Fri, 16 May 2008 10:09:11 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Mike Travis <travis@....com>
Subject: Re: [PATCH] modules: Use a better scheme for refcounting

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.
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