Re: [PATCH] memcg: avoid overflow in memcg_hierarchical_free_pages()

Daisuke Nishimura
2010-11-09T03:46:41Z

On Mon, 8 Nov 2010 17:15:20 -0800
Greg Thelen <gthelen@google.com> wrote:

> Use page counts rather than byte counts to avoid overflowing
> unsigned long local variables.
>
> Signed-off-by: Greg Thelen <gthelen@google.com>
> ---
> mm/memcontrol.c | 10 +++++-----
> 1 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6c7115d..b287afd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1345,17 +1345,17 @@ memcg_hierarchical_free_pages(struct mem_cgroup *mem)
> {
> unsigned long free, min_free;
>
hmm, the default value of RES_LIMIT is LLONG_MAX, so I think we must declare
"free" as unsinged long long to avoid overflow.

Thanks,
Daisuke Nishimura.

> - min_free = global_page_state(NR_FREE_PAGES) << PAGE_SHIFT;
> + min_free = global_page_state(NR_FREE_PAGES);
>
> while (mem) {
> - free = res_counter_read_u64(&mem->res, RES_LIMIT) -
> - res_counter_read_u64(&mem->res, RES_USAGE);
> + free = (res_counter_read_u64(&mem->res, RES_LIMIT) -
> + res_counter_read_u64(&mem->res, RES_USAGE)) >>
> + PAGE_SHIFT;
> min_free = min(min_free, free);
> mem = parent_mem_cgroup(mem);
> }
>
> - /* Translate free memory in pages */
> - return min_free >> PAGE_SHIFT;
> + return min_free;
> }
>
> /*
> --
> 1.7.3.1
>