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

Dave Hansen
2013-04-17T15:26:30Z

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 ;)