Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

[Jerry <uulinux@gmail.com>]

2013/7/19 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@gmail.com> wrote:
>
>> When PAGE_SHIFT > 20, the result of "20 - PAGE_SHIFT" is negative. The
>> calculating here will generate an unexpected result. In addition, if
>> PAGE_SHIFT > 20, The memory size represented by numentries was already
>> integral multiple of 1MB.
>>
>
> If you tell me that you have a machine which has PAGE_SIZE=2MB and this
> was the only problem which prevented Linux from running on that machine
> then I'll apply the patch ;)
>

Hi Morton:
I just "grep -rn "#define\s\+PAGE_SHIFT" arch/", and find the
PAGE_SHIFT in some architecture is very big.
such as the following in "arch/hexagon/include/asm/page.h"
....
#ifdef CONFIG_PAGE_SIZE_256KB
#define PAGE_SHIFT 18
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_256KB
#endif

#ifdef CONFIG_PAGE_SIZE_1MB
#define PAGE_SHIFT 20
#define HEXAGON_L1_PTE_SIZE __HVM_PDE_S_1MB
#endif
.....

Maybe the day of "A 2MB page" is not far. :-) I know it is just a
latent issue. Even if it won't generate a error when PAGE_SIZE == 20,
the calculating here is not necessary. In my mind, compiler would
optimize the calculating at that situation. But it is a little tricky.

In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
won't generate any machine instruction. Just a guarantee.

--
I love linux!!!

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