From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f70.google.com (mail-wm0-f70.google.com [74.125.82.70]) by kanga.kvack.org (Postfix) with ESMTP id 62E5D6B0069 for ; Fri, 9 Dec 2016 05:33:35 -0500 (EST) Received: by mail-wm0-f70.google.com with SMTP id y16so5175176wmd.6 for ; Fri, 09 Dec 2016 02:33:35 -0800 (PST) Received: from mout.kundenserver.de (mout.kundenserver.de. [212.227.17.24]) by mx.google.com with ESMTPS id ue16si33377516wjb.138.2016.12.09.02.33.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 09 Dec 2016 02:33:34 -0800 (PST) From: Arnd Bergmann Subject: Re: [RFC, PATCHv1 00/28] 5-level paging Date: Fri, 09 Dec 2016 11:24:12 +0100 Message-ID: <13962749.Q2mLWEctkQ@wuerfel> In-Reply-To: <20161209050130.GC2595@gmail.com> References: <20161208162150.148763-1-kirill.shutemov@linux.intel.com> <20161209050130.GC2595@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: owner-linux-mm@kvack.org List-ID: To: Ingo Molnar Cc: "Kirill A. Shutemov" , Linus Torvalds , Andrew Morton , x86@kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Dave Hansen , Andy Lutomirski , linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Ard Biesheuvel , maxim.kuvyrkov@linaro.org, Catalin Marinas , Will Deacon , broonie@kernel.org, schwidefsky@de.ibm.com On Friday, December 9, 2016 6:01:30 AM CET Ingo Molnar wrote: > > - Handle opt-in wider address space for userspace. > > > > Not all userspace is ready to handle addresses wider than current > > 47-bits. At least some JIT compiler make use of upper bits to encode > > their info. > > > > We need to have an interface to opt-in wider addresses from userspace > > to avoid regressions. > > > > For now, I've included testing-only patch which bumps TASK_SIZE to > > 56-bits. This can be handy for testing to see what breaks if we max-out > > size of virtual address space. > > So this is just a detail - but it sounds a bit limiting to me to provide an 'opt > in' flag for something that will work just fine on the vast majority of 64-bit > software. > > Please make this an opt out compatibility flag instead: similar to how we handle > address space layout limitations/quirks ABI details, such as ADDR_LIMIT_32BIT, > ADDR_LIMIT_3GB, ADDR_COMPAT_LAYOUT, READ_IMPLIES_EXEC, etc. We've had a similar discussion about JIT software on ARM64, which has a wide range of supported page table layouts and some software wants to limit that to a specific number. I don't remember the outcome of that discussion, but I'm adding a few people to Cc that might remember. There have also been some discussions in the past to make the depth of the page table a per-task decision on s390, since you may have some tasks that run just fine with two or three levels of paging while another task actually wants the full 64-bit address space. I wonder how much extra work this would be on top of the boot-time option. Arnd -- 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