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

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Message-ID: <516EC508.6070200@linux.intel.com>
Date:	Wed, 17 Apr 2013 08:51:36 -0700
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Dave Hansen <dave@...1.net>
CC:	zhang.yi20@....com.cn, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage



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
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