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

[Andrea Righi]

Andrew Morton wrote:
> On Thu, 12 Jun 2008 00:47:27 +0200
> Andrea Righi <righi.andrea@xxxxxxxxx> 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().

Good! Much simpler.

-Andrea
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