From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id 659046B0038 for ; Mon, 6 Nov 2017 03:25:37 -0500 (EST) Received: by mail-pg0-f69.google.com with SMTP id k7so11958220pga.8 for ; Mon, 06 Nov 2017 00:25:37 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id k8sor3348920pgo.33.2017.11.06.00.25.36 for (Google Transport Security); Mon, 06 Nov 2017 00:25:36 -0800 (PST) Date: Mon, 6 Nov 2017 19:25:24 +1100 From: Nicholas Piggin Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory Message-ID: <20171106192524.12ea3187@roar.ozlabs.ibm.com> In-Reply-To: <24b93038-76f7-33df-d02e-facb0ce61cd2@redhat.com> References: <20171105231850.5e313e46@roar.ozlabs.ibm.com> <871slcszfl.fsf@linux.vnet.ibm.com> <20171106174707.19f6c495@roar.ozlabs.ibm.com> <24b93038-76f7-33df-d02e-facb0ce61cd2@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Florian Weimer Cc: "Aneesh Kumar K.V" , "Kirill A. Shutemov" , linuxppc-dev@lists.ozlabs.org, linux-mm On Mon, 6 Nov 2017 09:11:37 +0100 Florian Weimer wrote: > On 11/06/2017 07:47 AM, Nicholas Piggin wrote: > > "You get < 128TB unless explicitly requested." > > > > Simple, reasonable, obvious rule. Avoids breaking apps that store > > some bits in the top of pointers (provided that memory allocator > > userspace libraries also do the right thing). > > So brk would simplify fail instead of crossing the 128 TiB threshold? Yes, that was the intention and that's what x86 seems to do. > > glibc malloc should cope with that and switch to malloc, but this code > path is obviously less well-tested than the regular way. Switch to mmap() I guess you meant? powerpc has a couple of bugs in corner cases, so those should be fixed according to intended policy for stable kernels I think. But I question the policy. Just seems like an ugly and ineffective wart. Exactly for such cases as this -- behaviour would change from run to run depending on your address space randomization for example! In case your brk happens to land nicely on 128TB then the next one would succeed. If not, then behaviour changes. And you didn't catch or prevent any bugs. Thanks, Nick -- 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