linux-kernel - Re: SLUB: Return ZERO_SIZE

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Message-Id: <20070601215427.f06d09e7.akpm@linux-foundation.org>
Date:	Fri, 1 Jun 2007 21:54:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	jeremy@...p.org
Subject: Re: SLUB: Return ZERO_SIZE_PTR for kmalloc(0)

On Fri, 1 Jun 2007 21:45:15 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Fri, 1 Jun 2007, Andrew Morton wrote:
> 
> > They are different instances which happen to have the same length (zero).
> 
> I guess one could use the slab allocators as a type of reservation 
> ticket generator with zero sized objects. Hmmm.... But is that really a 
> useful thing to do?
> 
> > But the code will incorrectly decide that they are the same instance.  It
> > might cause refcounting or accounting errors, for example.  I don't know - the
> > kernel's a big place.
> 
> That would have to occur with objects that are repeatedly allocated and 
> then linked toghether etc. Linking typicallty requires a listhead so its 
> typically difficult to do zero length objects.

Well I can't immediately think of a scenario in which it's likely to occur,
but we're in the position of trying to prove a negative.

Poke Bill Irwin - he'll think of something ;)

> > I agree the risk is low, but if something _does_ blow up, it will do so subtly.
> 
> The cases that we have seen so far are due to array allocations of N 
> elements where N == 0 leads to the creation of a zero sized object.
> The objects of the array are not zero sized it is just that zero of 
> them are allocated.

We lose leak-detection and double-free detection this way, too.  Not a big
deal.

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