linux-kernel - Re: [PATCH] Export symbol ksize()

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Date:	Sun, 15 Feb 2009 15:43:14 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-crypto@...r.kernel.org, Geert.Uytterhoeven@...ycom.com
Subject: Re: [PATCH] Export symbol ksize()

On Sun, 2009-02-15 at 13:36 -0800, Andrew Morton wrote:
> On Thu, 12 Feb 2009 17:55:04 +0200 Pekka Enberg <penberg@...helsinki.fi> wrote:
> 
> > On Thu, Feb 12, 2009 at 12:45:21PM +0200, Pekka Enberg wrote:
> > > > 
> > > > Because the API was being widely abused in the nommu code, for example.
> > > > I'd rather not add it back for this special case which can be handled
> > > > otherwise.
> > 
> > On Thu, 2009-02-12 at 18:50 +0800, Herbert Xu wrote:
> > > I'm sorry but that's like banning the use of heaters just because
> > > they can abused and cause fires.
> > > 
> > > I think I've said this to you before but in networking we very much
> > > want to use ksize because the standard case of a 1500-byte packet
> > > has loads of extra room given by kmalloc which all goes to waste
> > > right now.
> > > 
> > > If we could use ksize then we can stuff loads of metadata in that
> > > space.
> > 
> > OK, fair enough, I applied Kirill's patch. Thanks.
> > 
> 
> Could we please have more details regarding this:
> 
> > The ksize() function is not exported to modules because it has non-standard
> > behavour across different slab allocators. 
> 
> How does the behaviour differ?  It this documented?  Can we fix it?

SLAB and SLUB support calling ksize() on objects returned by
kmem_cache_alloc.

SLOB only supports it on objects from kmalloc. This is because it does
not store any size or type information in kmem_cache_alloc'ed objects.
Instead, it infers them from the cache argument.

Ideally SLAB and SLUB would complain about using ksize inappropriately
when debugging was enabled.

-- 
http://selenic.com : development and support for Mercurial and Linux


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