linux-kernel - Re: [patch v2] slub: add option to disable higher order debugging slabs

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Date:	Thu, 9 Jul 2009 16:26:36 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Larry Finger <Larry.Finger@...inger.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Johannes Berg <johannes@...solutions.net>
Subject: Re: [patch v2] slub: add option to disable higher order debugging
 slabs

On Tue, 7 Jul 2009, Christoph Lameter wrote:

> > + * Debugging flags that require metadata to be stored in the slab, up to
> > + * DEBUG_SIZE in size.
> > + */
> > +#define DEBUG_SIZE_FLAGS (SLAB_RED_ZONE | SLAB_POISON | SLAB_STORE_USER)
> > +#define DEBUG_SIZE (3 * sizeof(void *) + 2 * sizeof(struct track))
> 
> There is no need for DEBUG_SIZE since slub keeps both the size of the
> object kmem_cache->objsize and the size with the metadata kmem_cache->size
> 
> If the order of both is different then the order would increase.
> 

Without DEBUG_SIZE_FLAGS, the only way to determine what flags have 
increased the size is in calculate_sizes() and then disable them by 
default if slub_debug=O is specified.  calculate_sizes() is used by 
the `store', `poison', and `red_zone' callbacks, so the admin still has 
the ability to enable these options even though slub_debug=O was used.

So we can either mask off the size-increasing debug bits when the cache is 
created in kmem_cache_flags() like I did, or we can move the logic to 
calculate_sizes() with an added formal to determine whether this is from 
kmem_cache_open() or one of the attribute callbacks.

I think my solution is the cleanest and provides a single entity, 
DEBUG_SIZE_FLAGS, which specifies the flags that slub_debug=O clears if 
the minimum order increases.
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