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

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Message-Id: <200902240207.53590.nickpiggin@yahoo.com.au>
Date:	Tue, 24 Feb 2009 02:07:52 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Matt Mackall <mpm@...enic.com>,
	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,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [patch 1/7] slab: introduce kzfree()

On Tuesday 24 February 2009 01:51:05 Hugh Dickins wrote:
> On Tue, 24 Feb 2009, Nick Piggin wrote:
> > Well, the buffer is only non-modified in the case of one of the
> > allocators (SLAB). All others overwrite some of the data region
> > with their own metadata.
> >
> > I think it is OK to use const, though. Because k(z)free has the
> > knowledge that the data will not be touched by the caller any
> > longer.
>
> Sorry, you're not adding anything new to the thread here.
>
> Yes, the caller is surrendering the buffer, so we can get
> away with calling the argument const; and Linus argues that's
> helpful in the case of kfree (to allow passing a const pointer
> without having to cast it).

(Yes, not that I agree his argument is strong enough to be able
to call libc's definition wrong)

> My contention is that kzfree(const void *ptr) is nonsensical
> because it says please zero this buffer without modifying it.
>
> But the change has gone in, I seem to be the only one still
> bothered by it, and I've conceded that the "z" might stand
> for zap rather than zero.
>
> So it may be saying please hide the contents of this buffer,
> rather than please zero it.  And then it can be argued that
> the modification is an implementation detail which happens
> (like other housekeeping internal to the sl?b allocator)
> only after the original buffer has been freed.
>
> Philosophy.

Hmm, well it better if kzfree is defined to zap rather than zero
anyway. zap is a better definition because it theoretically allows
the implementation to do something else (poision it with some
other value; mark it as zapped and don't reallocate it without
zeroing it; etc). And also it doesn't imply that the caller still
cares about what it actually gets filled with.


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