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

[Jerry]

2013/7/20 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@xxxxxxxxx> wrote:
>
>> 2013/7/19 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
>> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@xxxxxxxxx> 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
>> .....
>
> Good heavens.
>
>> In my patch, I think compiler would optimize "if (20 > PAGE_SIZE)", it
>> won't generate any machine instruction. Just a guarantee.
>
> Well the existing code is a bit silly looking.  Why can't we just do
>
>         /* round applicable memory size up to nearest megabyte */
>         if (PAGE_SHIFT < 20)
>                 numentries = round_up(nr_kernel_pages, (1 << 20)/PAGE_SIZE);
>
> or similar?

Great. I have adjusted these code lines, and sent the latest one.

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