Re: [Kernel-janitors] mm/slab.c: linux 2.6.1 fix 2 unguarded kmalloc and a PAGE_SHIFT

[Matthew Wilcox <willy@debian.org>]

On Sun, Jan 25, 2004 at 03:02:24PM +0100, Walter Harms wrote:
> Hi list,
> this fixes catches 2 unguarded kmallocs() and changes a statement so that PAGE_SHIFT >20 causes a warning. 
> At least sparc64 is prepared for a  PAGE_SHIFT >20.

> -       if (num_physpages > (32 << 20) >> PAGE_SHIFT)
> +       if (num_physpages > (32 << (20-PAGE_SHIFT) )

Erm, that's a bad change to make.  There's nothing wrong with having
a PAGE_SHIFT of >20, and this code would still work ... but with your
change it's now undefined behaviour.  Please don't make this change.

(worked examples:

1. suppose we have a PAGE_SHIFT of 24 (16MB page size), the original
code shifts 32 left by 20 then right by 24, net result a right shift of
4 => 2.  Since any machine with a 16MB page size is going to have more
than 2 pages, we're fine.  the new code will shift 32 left by -4 which
is undefined.  bad.

2. suppose we have a PAGE_SHIFT of 26 (64MB page size), the original
code shifts right by 6, result 0.  Since the number of pages is > 0
(one hopes ;-), the test will also succeed as desired.

)

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