linux-kernel - Re: Uninline kcalloc

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Message-Id: <07635976-4FC3-4E14-9B94-CE4D6446FD4E@gmail.com>
Date:	Wed, 15 Feb 2012 14:14:25 -0500
From:	Xi Wang <xi.wang@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Jesper Juhl <jj@...osbits.net>, Jens Axboe <axboe@...nel.dk>,
	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: Uninline kcalloc

On Feb 14, 2012, at 5:08 PM, Christoph Lameter wrote:
> kcalloc is still there. Certainly useful for legacy purposes. But I'd feel
> better if I had fine grained control over the size of my allocation rather
> than rely on the slab allocators to check up on my multiplication.
> 
> With these patches both is possible. And if you want the check of an
> allocation that is not zeroed then you can do so because you have a
> function that will perform the size check for you without calling into the
> slab allocator.

In the code you proposed, where calculate_array_size() returns 0
for overflow, one has to write:

        size_t s = calculate_array_size(n, size);
        if (s)
                p = kmalloc(s, ...);

This "if" thing is just too verbose --- you need three lines to
allocate an array.

We could change calculate_array_size() to return ULONG_MAX or some
large number with which kmalloc() would fail.  Then one would write:

        p = kmalloc(calculate_array_size(n, size), ...);

This looks better to me.  The advantage is that we don't need another
allocator (and avoid this name picking game).  The disadvantage is
that the semantics of calculate_array_size(), returning ULONG_MAX
on overflow, sounds sort of strange.

- xi

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