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

Andrea Righi
2008-06-11T22:47:31Z

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.

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; \
})

-Andrea