linux-kernel - Re: [PATCH 2/2] SLUB: Disable debugging if it increases the minimum page order

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Date:	Thu, 11 Jun 2009 09:43:47 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Pekka J Enberg <penberg@...helsinki.fi>
cc:	linux-kernel@...r.kernel.org, mel@....ul.ie,
	Larry.Finger@...inger.net
Subject: Re: [PATCH 2/2] SLUB: Disable debugging if it increases the minimum
 page order

We have had that with SLAB. NO! This leads to the situation that some
slabs have debug on and some have not. You just do not know which.

Note that CONFIG_SLUB_DEBUG only enables the code to debug a slab. It does
not enable debugging for each slab. CONFIG_SLUB_DEBUG_ON does that.

On Thu, 11 Jun 2009, Pekka J Enberg wrote:

> From: Pekka Enberg <penberg@...helsinki.fi>
>
> As CONFIG_SLUB_DEBUG is almost always enabled, we need to disable debugging on
> per-cache basis if the added debug metadata increases minimum page order. This
> is a problem for large caches such as kmalloc-4096 as seen in the following
> bugzilla report:
>
>   http://bugzilla.kernel.org/show_bug.cgi?id=13319
>
> Cc: Christoph Lameter <cl@...ux-foundation.org>
> Cc: Larry Finger <Larry.Finger@...inger.net>
> Cc: Mel Gorman <mel@....ul.ie>
> Signed-off-by: Pekka Enberg <penberg@...helsinki.fi>
> ---
>  mm/slub.c |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 2bbacfc..45080d3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2385,6 +2385,16 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>
>  }
>
> +#define MAX_DEBUG_SIZE (3 * sizeof(void *) + 2 * sizeof(struct track))
> +
> +static bool must_disable_debug(size_t size)
> +{
> +	/*
> +	 * Disable debugging if it increases the minimum page order.
> +	 */
> +	return get_order(size + MAX_DEBUG_SIZE) > get_order(size);
> +}
> +
>  static int kmem_cache_open(struct kmem_cache *s, gfp_t gfpflags,
>  		const char *name, size_t size,
>  		size_t align, unsigned long flags,
> @@ -2397,6 +2407,9 @@ static int kmem_cache_open(struct kmem_cache *s, gfp_t gfpflags,
>  	s->align = align;
>  	s->flags = kmem_cache_flags(size, flags, name, ctor);
>
> +	if (must_disable_debug(size))
> +		s->flags &= ~(SLAB_POISON|SLAB_RED_ZONE|SLAB_STORE_USER);
> +
>  	if (!calculate_sizes(s, -1))
>  		goto error;
>
>
--
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