linux-kernel - Re: SLUB: Return ZERO_SIZE

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Date:	Fri, 1 Jun 2007 21:45:15 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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, 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.

> 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.
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