Re: [PATCH 1/1] kernel/fork.c: avoid division by zero

[Guenter Roeck <linux@roeck-us.net>]

On 02/16/2015 10:45 AM, Heinrich Schuchardt wrote:
> I am resending this mail because the linux-kernel list was not CCed.
>
Yes, sorry, that got lost. The "forward" feature on lkml.org doesn't work
as well as it used to, unfortunately, and I keep forgetting.

> On 16.02.2015 19:23, Guenter Roeck wrote:
>> On Mon, Feb 16, 2015 at 07:00:44PM +0100, Heinrich Schuchardt wrote:
>>> PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
>>> THREAD_SIZE.
>>>
>>> E.g. architecture hexagon may have page size 1M and thread size 4096.
>>>
>>> This would lead to a division by zero.
>>>
>>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
>>> ---
>>>   kernel/fork.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index cf65139..71e2583 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -273,7 +273,8 @@ void __init fork_init(unsigned long mempages)
>>>   	 * value: the thread structures can take up at most half
>>>   	 * of memory.
>>>   	 */
>>> -	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
>>> +	max_threads = (u64) PAGE_SIZE * (u64) mempages
>>> +		    / (u64) THREAD_SIZE / 8UL;
>>
>> How about
>> 	max_threads = mempages / (8 * THREAD_SIZE) * PAGE_SIZE;
>>
>> instead ? That would avoid the u64.
>
> We have to consider the case mempages < 8 * THREAD_SIZE.
>
> Let
>    mempages = 1024
>    THREAD_SIZE = 4096
>    PAGE_SIZE = 1M
>
> Your code gives 0.
> Mine gives 8192.
>
Makes sense.

Thanks,
Guenter