Re: [PATCH] kernel: sysctl: use 'unsigned long' type for 'zero' variable

[Andrey Ryabinin <a.ryabinin@samsung.com>]

On 12/04/2014 03:19 AM, Andrew Morton wrote:
> On Wed, 3 Dec 2014 15:25:24 -0800 Andrew Morton <akpm@linux-foundation.org> wrote:
> 
>> On Wed, 03 Dec 2014 15:41:21 +0300 Andrey Ryabinin <a.ryabinin@samsung.com> wrote:
>>
>>>
>>> Use the 'unsigned long' type for 'zero' variable to fix this.
>>> Changing type to 'unsigned long' shouldn't affect any other users
>>> of this variable.
>>>
>>> Reported-by: Dmitry Vyukov <dvyukov@google.com>
>>> Fixes: ed4d4902ebdd ("mm, hugetlb: remove hugetlb_zero and hugetlb_infinity")
>>> Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
>>> ---
>>>  kernel/sysctl.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>>> index 15f2511..45c45c9 100644
>>> --- a/kernel/sysctl.c
>>> +++ b/kernel/sysctl.c
>>> @@ -120,7 +120,7 @@ static int sixty = 60;
>>>  
>>>  static int __maybe_unused neg_one = -1;
>>>  
>>> -static int zero;
>>> +static unsigned long zero;
>>>  static int __maybe_unused one = 1;
>>>  static int __maybe_unused two = 2;
>>>  static int __maybe_unused four = 4;
>>
>> Yeah, this is ghastly.
>>
>> Look at 
>>
>> 	{
>> 		.procname	= "numa_balancing",
>> 		.data		= NULL, /* filled in by handler */
>> 		.maxlen		= sizeof(unsigned int),
>> 		.mode		= 0644,
>> 		.proc_handler	= sysctl_numa_balancing,
>> 		.extra1		= &zero,
>> 		.extra2		= &one,
>> 	},
>>
>> Now extra1 points at a long and extra2 points at an int. 
>> sysctl_numa_balancing() calls proc_dointvec_minmax() and I think your
>> patch just broke big-endian 64-bit machines.  "sched_autogroup_enabled"
>> breaks as well.
> 
> Taking another look at this...
> 
> numa_balancing will continue to work on big-endian because of course
> zero is still zero when byteswapped.  But that's such a hack, isn't
> documented and doesn't work for "one", "sixty", etc.
> 

Yeah, I agree it's a bit hacky.

> I'm thinking a better fix here is to switch hugetlb_sysctl_handler to
> use `int's.  2^32 hugepages is enough for anybody.
> 

It's 8 petabytes for 2MB pages, so yeah should be enough.
Perhaps it also makes sense to change types for counters in 'struct hstate' from longs to ints.



> hugetlb_overcommit_handler() will need conversion also.
> 
> Perhaps auditing all the proc_doulongvec_minmax callsites is the way to
> attack this.
> 

I've looked through this yesterday and didn't found anything obviously wrong.
Though I could easily miss something.