Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752210AbdLBStp (ORCPT ); Sat, 2 Dec 2017 13:49:45 -0500 Received: from mail-oi0-f65.google.com ([209.85.218.65]:44005 "EHLO mail-oi0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751806AbdLBStm (ORCPT ); Sat, 2 Dec 2017 13:49:42 -0500 X-Google-Smtp-Source: AGs4zMaN7chGsn1l/xeibuW9n6kjeVxsti9bbP5IfUcidCz0cDIbVXMwhrnvUp3GAG1Q5Rlft5rK4ZKUTd8wLf0HhSk= MIME-Version: 1.0 In-Reply-To: <20171202150554.GA30203@bombadil.infradead.org> References: <20171202021626.26478-1-jhubbard@nvidia.com> <20171202150554.GA30203@bombadil.infradead.org> From: Jann Horn Date: Sat, 2 Dec 2017 19:49:20 +0100 Message-ID: Subject: Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged To: Matthew Wilcox Cc: john.hubbard@gmail.com, Michael Kerrisk , linux-man , Linux API , Michael Ellerman , linux-mm@kvack.org, LKML , linux-arch , Michal Hocko , John Hubbard Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1305 Lines: 24 On Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox wrote: > On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@gmail.com wrote: >> MAP_FIXED has been widely used for a very long time, yet the man >> page still claims that "the use of this option is discouraged". > > I think we should continue to discourage the use of this option, but > I'm going to include some of your text in my replacement paragraph ... > > -Because requiring a fixed address for a mapping is less portable, > -the use of this option is discouraged. > +The use of this option is discouraged because it forcibly unmaps any > +existing mapping at that address. Programs which use this option need > +to be aware that their memory map may change significantly from one run to > +the next, depending on library versions, kernel versions and random numbers. How about adding something explicit about when it's okay to use MAP_FIXED? "This option should only be used to displace an existing mapping that is controlled by the caller, or part of such a mapping." or something like that? > +In a threaded process, checking the existing mappings can race against > +a new dynamic library being loaded malloc() and its various callers can also cause mmap() calls, which is probably more relevant than library loading.