linux-kernel - Re: [PATCH] mm: negative left shift count when PAGE

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Date:	Fri, 19 Jul 2013 14:57:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jerry <uulinux@...il.com>
Cc:	zhuwei.lu@...hermind.com, tianfu.huang@...hermind.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: negative left shift count when PAGE_SHIFT > 20

On Fri, 19 Jul 2013 07:47:02 +0800 Jerry <uulinux@...il.com> wrote:

> 2013/7/19 Andrew Morton <akpm@...ux-foundation.org>:
> > On Fri, 19 Jul 2013 00:56:12 +0800 Jerry <uulinux@...il.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
> .....

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?
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