From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx113.postini.com [74.125.245.113]) by kanga.kvack.org (Postfix) with SMTP id 0AE8D6B00A7 for ; Wed, 17 Apr 2013 11:51:37 -0400 (EDT) Message-ID: <516EC508.6070200@linux.intel.com> Date: Wed, 17 Apr 2013 08:51:36 -0700 From: Darren Hart MIME-Version: 1.0 Subject: Re: [PATCH] futex: bugfix for futex-key conflict when futex use hugepage References: <516EAF31.8000107@linux.intel.com> <516EBF23.2090600@sr71.net> In-Reply-To: <516EBF23.2090600@sr71.net> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: zhang.yi20@zte.com.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Dave Hansen 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org