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SubjectRe: mem_cgroup_get_limit() return type, was [patch] memcg: fix unit mismatch in memcg oom limit calculation
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David Rientjes <rientjes@google.com> writes:

> On Tue, 9 Nov 2010, Greg Thelen wrote:
>
>> >> > Adding the number of swap pages to the byte limit of a memory control
>> >> > group makes no sense. Convert the pages to bytes before adding them.
>> >> >
>> >> > The only user of this code is the OOM killer, and the way it is used
>> >> > means that the error results in a higher OOM badness value. Since the
>> >> > cgroup limit is the same for all tasks in the cgroup, the error should
>> >> > have no practical impact at the moment.
>> >> >
>> >> > But let's not wait for future or changing users to trip over it.
>> >>
>> >> Thanks for the fix.
>> >>
>> >
>> > Nice catch, but it's done in the opposite way: the oom killer doesn't use
>> > byte limits but page limits. So this needs to be
>> >
>> > (res_counter_read_u64(&memcg->res, RES_LIMIT) >> PAGE_SHIFT) +
>> > total_swap_pages;
>>
>> In -mm, the oom killer queries memcg for a byte limit using
>> mem_cgroup_get_limit(). The following is from
>> mem_cgroup_out_of_memory():
>>
>> limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
>>
>
> Oops, I missed that. I think Johannes' patch is better because
> mem_cgroup_get_limit() may eventually be used elsewhere and the subsystem
> has byte granularity.

I have no problem with mem_cgroup_get_limit() returning a byte count as
you prefer. I think this approach does have an issue.
"mem_cgroup_get_limit() >> PAGE_SHIFT" may not fit within unsigned long
on 32-bit machines. Does this cause a problem for the 'limit' local
variable in mem_cgroup_out_of_memory():
limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;

Do we need something like the following in mem_cgroup_out_of_memory() to
guard against this overflow?
limit = min(mem_cgroup_get_limit(mem) >> PAGE_SHIFT, ULONG_MAX);


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