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SubjectRe: [-mm][PATCH 2/4] Setup the memrlimit controller (v5)
On Wed, 11 Jun 2008 22:17:12 +0200
Andrea Righi <righi.andrea@gmail.com> wrote:

> Andrew Morton wrote:
> > On Wed, 11 Jun 2008 19:10:40 +0200 (MEST)
> > Andrea Righi <righi.andrea@gmail.com> wrote:
> >
> >> Balbir Singh wrote:
> >>> +static int memrlimit_cgroup_write_strategy(char *buf, unsigned long long *tmp)
> >>> +{
> >>> + *tmp = memparse(buf, &buf);
> >>> + if (*buf != '\0')
> >>> + return -EINVAL;
> >>> +
> >>> + *tmp = PAGE_ALIGN(*tmp);
> >>> + return 0;
> >>> +}
> >> We shouldn't use PAGE_ALIGN() here, otherwise we limit the address space
> >> to 4GB on 32-bit architectures (that could be reasonable, because this
> >> is a per-cgroup limit and not per-process).
> >>
> >> Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
> >> ---
> >> mm/memrlimitcgroup.c | 4 +++-
> >> 1 files changed, 3 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/mm/memrlimitcgroup.c b/mm/memrlimitcgroup.c
> >> index 9a03d7d..2d42ff3 100644
> >> --- a/mm/memrlimitcgroup.c
> >> +++ b/mm/memrlimitcgroup.c
> >> @@ -29,6 +29,8 @@
> >> #include <linux/res_counter.h>
> >> #include <linux/memrlimitcgroup.h>
> >>
> >> +#define PAGE_ALIGN64(addr) (((((addr)+PAGE_SIZE-1))>>PAGE_SHIFT)<<PAGE_SHIFT)
> >> +
> >> struct cgroup_subsys memrlimit_cgroup_subsys;
> >>
> >> struct memrlimit_cgroup {
> >> @@ -124,7 +126,7 @@ static int memrlimit_cgroup_write_strategy(char *buf, unsigned long long *tmp)
> >> if (*buf != '\0')
> >> return -EINVAL;
> >>
> >> - *tmp = PAGE_ALIGN(*tmp);
> >> + *tmp = PAGE_ALIGN64(*tmp);
> >> return 0;
> >> }
> >>
> >
> > I don't beleive the change is needed.
> >
> > #define PAGE_ALIGN(addr) (((addr)+PAGE_SIZE-1)&PAGE_MASK)
> >
> > that implementation will behaved as desired when passed a 64-bit addr?
>
> If I'm not doing something wrong, here is what happens on my i386 box:
>
> $ uname -m
> i686
> $ cat 64-bit-page-align.c
> #include <stdio.h>
> #include <asm/page.h>
>
> #define PAGE_ALIGN(addr) (((addr)+PAGE_SIZE-1)&PAGE_MASK)
> #define PAGE_ALIGN64(addr) (((((addr)+PAGE_SIZE-1))>>PAGE_SHIFT)<<PAGE_SHIFT)
>
> #define SIZE ((1ULL << 32) - 1)
>
> int main(int argc, char **argv)
> {
> unsigned long long good, bad;
>
> good = (unsigned long long)PAGE_ALIGN64(SIZE);
> bad = (unsigned long long)PAGE_ALIGN(SIZE);
>
> fprintf(stdout, "good = %llu, bad = %llu\n", good, bad);
>
> return 0;
> }
> $ gcc -O2 -o 64-bit-page-align 64-bit-page-align.c
> $ ./64-bit-page-align
> good = 4294967296, bad = 0
> ^^^^^^^
> On a x86_64, instead, both PAGE_ALIGN()s work as expected:

That's weird. We have an expression which contains a combination of UL
constants and ULL constants, and the compiler _isn't_ converting
everything into ULL.


> $ uname -m
> x86_64
> $ gcc -O2 -o 64-bit-page-align 64-bit-page-align.c
> $ ./64-bit-page-align
> good = 4294967296, bad = 4294967296
>
> 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.

If we go this way then first we should hoist the PAGE_ALIGN definitions
out of include/asm-*/page.h and into, umm, include/linux/mm.h.
That might cause build errors but from a bit of grepping around, only
include/asm-arm/cacheflush.h is at risk, and it already includes
linux/mm.h.


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