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

Greg Thelen
2010-11-09T03:52:45Z

On Mon, Nov 8, 2010 at 7:44 PM, Daisuke Nishimura
<nishimura@mxp.nes.nec.co.jp> wrote:
> 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.

Agreed. I am testing a fix for that issue now. I do not want
complicate this patch with the RES_LIMIT issue you mention. The fix
will be in a separate patch.

> 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
>>
>