From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 28FF66B0033 for ; Mon, 4 Dec 2017 21:14:21 -0500 (EST) Received: by mail-pf0-f198.google.com with SMTP id 8so14741103pfv.12 for ; Mon, 04 Dec 2017 18:14:21 -0800 (PST) Received: from hqemgate15.nvidia.com (hqemgate15.nvidia.com. [216.228.121.64]) by mx.google.com with ESMTPS id f80si11108351pfa.193.2017.12.04.18.14.19 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 04 Dec 2017 18:14:19 -0800 (PST) Subject: Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation References: <20171204021411.4786-1-jhubbard@nvidia.com> <20171204105549.GA31332@rei> From: John Hubbard Message-ID: Date: Mon, 4 Dec 2017 18:14:18 -0800 MIME-Version: 1.0 In-Reply-To: <20171204105549.GA31332@rei> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Cyril Hrubis Cc: Michael Kerrisk , linux-man , linux-api@vger.kernel.org, Michael Ellerman , linux-mm@kvack.org, LKML , linux-arch@vger.kernel.org, Jann Horn , Matthew Wilcox , Michal Hocko On 12/04/2017 02:55 AM, Cyril Hrubis wrote: > Hi! > I know that we are not touching the rest of the existing description for > MAP_FIXED however the second sentence in the manual page says that "addr > must be a multiple of the page size." Which however is misleading as > this is not enough on some architectures. Code in the wild seems to > (mis)use SHMLBA for aligment purposes but I'm not sure that we should > advise something like that in the manpages. > > So what about something as: > > "addr must be suitably aligned, for most architectures multiple of page > size is sufficient, however some may impose additional restrictions for > page mapping addresses." > Hi Cyril, Right, so I've been looking into this today, and I think we can go a bit further than that, even. The kernel, as far back as the *original* git commit in 2005, implements mmap on ARM by requiring that the address is aligned to SHMLBA: arch/arm/mm/mmap.c:50: if (flags & MAP_FIXED) { if (aliasing && flags & MAP_SHARED && (addr - (pgoff << PAGE_SHIFT)) & (SHMLBA - 1)) return -EINVAL; return addr; } So, given that this has been the implementation for the last 12+ years (and probably the whole time, in fact), I think we can be bold enough to use this wording for the second sentence of MAP_FIXED: "addr must be a multiple of SHMLBA (), which in turn is either the system page size (on many architectures) or a multiple of the system page size (on some architectures)." What do you think? thanks, John Hubbard NVIDIA > Which should at least hint the reader that this is architecture specific. > -- 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