From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f180.google.com (mail-pf0-f180.google.com [209.85.192.180]) by kanga.kvack.org (Postfix) with ESMTP id 664456B0038 for ; Mon, 14 Dec 2015 15:51:55 -0500 (EST) Received: by pff63 with SMTP id 63so17235174pff.2 for ; Mon, 14 Dec 2015 12:51:55 -0800 (PST) Received: from mail-pa0-x22c.google.com (mail-pa0-x22c.google.com. [2607:f8b0:400e:c03::22c]) by mx.google.com with ESMTPS id g87si19316017pfj.194.2015.12.14.12.51.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 14 Dec 2015 12:51:54 -0800 (PST) Received: by pacwq6 with SMTP id wq6so109440605pac.1 for ; Mon, 14 Dec 2015 12:51:54 -0800 (PST) Subject: Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS. References: <1449856338-30984-1-git-send-email-dcashman@android.com> <1449856338-30984-2-git-send-email-dcashman@android.com> <1449856338-30984-3-git-send-email-dcashman@android.com> <1449856338-30984-4-git-send-email-dcashman@android.com> <1449856338-30984-5-git-send-email-dcashman@android.com> <566F1154.7030703@zytor.com> From: Daniel Cashman Message-ID: <566F2BE7.4090904@android.com> Date: Mon, 14 Dec 2015 12:51:51 -0800 MIME-Version: 1.0 In-Reply-To: <566F1154.7030703@zytor.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "H. Peter Anvin" , linux-kernel@vger.kernel.org Cc: linux@arm.linux.org.uk, akpm@linux-foundation.org, keescook@chromium.org, mingo@kernel.org, linux-arm-kernel@lists.infradead.org, corbet@lwn.net, dzickus@redhat.com, ebiederm@xmission.com, xypron.glpk@gmx.de, jpoimboe@redhat.com, kirill.shutemov@linux.intel.com, n-horiguchi@ah.jp.nec.com, aarcange@redhat.com, mgorman@suse.de, tglx@linutronix.de, rientjes@google.com, linux-mm@kvack.org, linux-doc@vger.kernel.org, salyzyn@android.com, jeffv@google.com, nnk@google.com, catalin.marinas@arm.com, will.deacon@arm.com, x86@kernel.org, hecmargi@upv.es, bp@suse.de, dcashman@google.com, arnd@arndb.de, jonathanh@nvidia.com On 12/14/2015 10:58 AM, H. Peter Anvin wrote: > On 12/11/15 09:52, Daniel Cashman wrote: >> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c >> index 844b06d..647fecf 100644 >> --- a/arch/x86/mm/mmap.c >> +++ b/arch/x86/mm/mmap.c >> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void) >> { >> unsigned long rnd; >> >> - /* >> - * 8 bits of randomness in 32bit mmaps, 20 address space bits >> - * 28 bits of randomness in 64bit mmaps, 40 address space bits >> - */ >> if (mmap_is_ia32()) >> - rnd = (unsigned long)get_random_int() % (1<<8); >> +#ifdef CONFIG_COMPAT >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits); >> +#else >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> +#endif >> else >> - rnd = (unsigned long)get_random_int() % (1<<28); >> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits); >> >> return rnd << PAGE_SHIFT; >> } >> > > Now, you and I know that both variants can be implemented with a simple > AND, but I have a strong suspicion that once this is turned into a > variable, this will in fact be changed from an AND to a divide. > > So I'd prefer to use the > "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead. Good point. Will change in v7 across patch-set. Thank You, Dan -- 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