linux-kernel - Re: [-mm][PATCH 2/4] Setup the memrlimit controller (v5)

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Date:	Wed, 11 Jun 2008 15:55:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	righi.andrea@...il.com
Cc:	balbir@...ux.vnet.ibm.com, linux-mm@...ck.org,
	skumar@...ux.vnet.ibm.com, yamamoto@...inux.co.jp,
	menage@...gle.com, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, xemul@...nvz.org,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [-mm][PATCH 2/4] Setup the memrlimit controller (v5)

On Thu, 12 Jun 2008 00:47:27 +0200
Andrea Righi <righi.andrea@...il.com> wrote:

> Andrew Morton wrote:
> >> At least we could add something like:
> >>
> >> #ifdef CONFIG_32BIT
> >> #define PAGE_ALIGN64(addr) (((((addr)+PAGE_SIZE-1))>>PAGE_SHIFT)<<PAGE_SHIFT)
> >> #else
> >> #define PAGE_ALIGN64(addr) PAGE_ALIGN(addr)
> >> #endif
> >>
> >> But IMHO the single PAGE_ALIGN64() implementation is more clear.
> > 
> > No, we should just fix PAGE_ALIGN.  It should work correctly when
> > passed a long-long.  Otherwse it's just a timebomb.
> > 
> > This:
> > 
> > #define PAGE_ALIGN(addr) ({				\
> > 	typeof(addr) __size = PAGE_SIZE;		\
> > 	typeof(addr) __mask = PAGE_MASK;		\
> > 	(addr + __size - 1) & __mask;			\
> > })
> > 
> > (with a suitable comment) does what we want.  I didn't check to see
> > whether this causes the compiler to generate larger code, but it
> > shouldn't.
> > 
> 
> No, it doesn't work. The problem seems to be in the PAGE_MASK definition
> (from include/asm-x86/page.h for example):
> 
> /* PAGE_SHIFT determines the page size */
> #define PAGE_SHIFT      12
> #define PAGE_SIZE       (_AC(1,UL) << PAGE_SHIFT)
> #define PAGE_MASK       (~(PAGE_SIZE-1))
> 
> The "~" is performed on a 32-bit value, so everything in "and" with
> PAGE_MASK greater than 4GB will be truncated to the 32-bit boundary.

OK, I oversimplified my testcase.

> What do you think about the following?
> 
> #define PAGE_SIZE64 (1ULL << PAGE_SHIFT)
> #define PAGE_MASK64 (~(PAGE_SIZE64 - 1))
> 
> #define PAGE_ALIGN(addr) ({					\
> 	typeof(addr) __size = PAGE_SIZE;			\
> 	typeof(addr) __ret = (addr) + __size - 1;		\
> 	__ret > -1UL ? __ret & PAGE_MASK64 : __ret & PAGE_MASK;	\
> })

Complex.  And I'd worry about added code overhead.

What about

#define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)

?

afaict ALIGN() tries to do the right thing, and if it doesn't, we
should fix ALIGN().

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