linux-kernel - Re: [RFC/PATCH 1/3] SLAB: Add PageSlab checking to ksize()

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Date:	Thu, 22 May 2008 13:31:56 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Paul Mundt <lethal@...ux-sh.org>,
	Christoph Lameter <clameter@....com>,
	linux-kernel@...r.kernel.org, dhowells@...hat.com
Subject: Re: [RFC/PATCH 1/3] SLAB: Add PageSlab checking to ksize()


On Thu, 2008-05-22 at 19:13 +0300, Pekka Enberg wrote:
> Hi Matt,
> 
> On Thu, May 22, 2008 at 6:01 PM, Matt Mackall <mpm@...enic.com> wrote:
> > As I've said several times, ksize() on kmem_cache_alloced objects
> > -cannot work- on SLOB. Calling ksize() on something returned by
> > kmem_cache_alloc is a categorical error.
> 
> Well, it's a historical fact that ksize() worked for both kmalloc()
> and kmem_cache_alloc() (see the kernedoc comment in mm/slab.c).

Indeed. It looks like it was in fact introduced for nommu (back in
2.5.47). But much like kfree(kmem_cache_alloc()) is a bogus thing to do,
ksize(kmem_cache_alloc()) is assuming too much about the relationship
between kmalloc and kmem_cache_alloc.

Nommu's accounting code makes two misguided assumptions a) that we can
determine how/whether something was allocated just from a pointer b)
that the size of that object can be determined dynamically in any case
other than kmalloc. But it really shouldn't need to do either of these.

> However, I think we should just look at getting rid of ksize()
> altogether as it's only (ab)used by the nommu code and few call-sites
> that open-code krealloc().

Right.

-- 
Mathematics is the supreme nostalgia of our time.

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