linux-kernel - Re: [patch 1/7] slab: introduce kzfree()

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Message-ID: <Pine.LNX.4.64.0902191616250.8594@blonde.anvils>
Date:	Thu, 19 Feb 2009 16:34:41 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Vrabel <david.vrabel@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chas Williams <chas@....nrl.navy.mil>,
	Evgeniy Polyakov <johnpol@....mipt.ru>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [patch 1/7] slab: introduce kzfree()

On Thu, 19 Feb 2009, Pekka Enberg wrote:
> On Wed, 2009-02-18 at 10:50 +0000, David Vrabel wrote:
> > > > Johannes Weiner wrote:
> > > > > +void kzfree(const void *p)
> > > > 
> > > > Shouldn't this be void * since it writes to the memory?
> > > 
> > > No. kfree() writes to the memory as well to update freelists, poisoning
> > > and such so kzfree() is not at all different from it.
> 
> On Thu, 2009-02-19 at 10:22 +0900, KOSAKI Motohiro wrote:
> > I don't think so. It's debetable thing.
> > 
> > poisonig is transparent feature from caller.
> > but the caller of kzfree() know to fill memory and it should know.
> 
> Debatable, sure, but doesn't seem like a big enough reason to make
> kzfree() differ from kfree().

There may be more important things for us to worry about,
but I do strongly agree with KOSAKI-san on this.

kzfree() already differs from kfree() by a "z": that "z" says please
zero the buffer pointed to; "const" says it won't modify the buffer
pointed to.  What sense does kzfree(const void *) make?  Why is
keeping the declarations the same apart from the "z" desirable?

By all means refuse to add kzfree(), but please don't add it with const.

I can see that the "const" in kfree(const void *) is debatable
[looks to see how userspace free() is defined: without a const],
I can see that it might be nice to have some "goesaway" attribute
for such pointers instead; but I don't see how you can argue for
kzalloc(const void *).

Hugh
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