linux-kernel - Re: SLUB: Return ZERO_SIZE

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Message-ID: <alpine.LFD.0.98.0706041143240.23741@woody.linux-foundation.org>
Date:	Mon, 4 Jun 2007 11:47:18 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: SLUB: Return ZERO_SIZE_PTR for kmalloc(0)

On Fri, 1 Jun 2007, Christoph Lameter wrote:
>
> Instead of returning the smallest available object return ZERO_SIZE_PTR.

Ok, I just noticed that this still has a bug: not just kfree(), but 
krealloc() needs to treat ZERO_SIZE_PTR properly.

Your patch introduces two bugs in mm/slub.c:krealloc():

 - The

	if (unlikely(!p))
		return kmalloc(new_size, flags);

   test needs to be for NULL or ZERO_SIZE_PTR. Otherwise it will oops in 
   ksize(p), I think.

 - And the

	if (unlikely(!new_size)) {
		kfree(p);
		return NULL;
	}

   thing should logically return ZERO_SIZE_PTR instead of NULL.

So basically

	krealloc(kmalloc(0), n, flags);

must work, and

	krealloc(old, 0, flags)

should return a zero-sized allocation.

I'd forgotten about krealloc(), because that whole concept is fairly new.

		Linus
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