Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage

Darren Hart
2013-04-17T15:51:38Z

On 04/17/2013 08:26 AM, Dave Hansen wrote:
> On 04/17/2013 07:18 AM, Darren Hart wrote:
>>>> This also needs a comment in futex.h describing the usage of the offset
>>>> field in union futex_key as well as above get_futex_key describing the
>>>> key for shared mappings.
>>>>
>>> As far as I know , the max size of one hugepage is 1 GBytes for x86 cpu.
>>> Can some other cpus support greater hugepage even more than 4 GBytes? If
>>> so, we can change the type of 'offset' from int to long to avoid
>>> truncating.
>>
>> I discussed this with Dave Hansen, on CC, and he thought we needed 9
>> bits, so even on x86 32b we should be covered.
>
> I think the problem is actually on 64-bit since you still only have
> 32-bits in an 'int' there.
>
> I guess it's remotely possible that we could have some
> mega-super-huge-gigantic pages show up in hardware some day, or that
> somebody would come up with software-only one. I bet there's a lot more
> code that will break in the kernel than this futex code, though.
>
> The other option would be to start #defining some build-time constant
> for what the largest possible huge page size is, then BUILD_BUG_ON() it.
>
> Or you can just make it a long ;)

If we make it a long I'd want to see futextest performance tests before
and after. Messing with the futex_key has been known to have bad results
in the past :-)

--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel