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

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Message-Id: <1247208879.19026.2.camel@penberg-laptop>
Date:	Fri, 10 Jul 2009 09:54:39 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	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.

On Thu, 2009-07-09 at 16:26 -0700, David Rientjes wrote:
> 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.

Yup, agreed. I applied the patch, thanks everyone!

			Pekka

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