From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [PATCH] Inconsistent mmap()/mremap() flags From: Thayne Harbaugh Reply-To: thayne@c2.net In-Reply-To: <20071002051526.GA29615@one.firstfloor.org> References: <1190958393.5128.85.camel@phantasm.home.enterpriseandprosperity.com> <200710011313.30171.andi@firstfloor.org> <1191293830.5200.22.camel@phantasm.home.enterpriseandprosperity.com> <20071002051526.GA29615@one.firstfloor.org> Content-Type: text/plain Date: Tue, 02 Oct 2007 01:06:12 -0600 Message-Id: <1191308772.5200.66.camel@phantasm.home.enterpriseandprosperity.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Andi Kleen Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, discuss@x86-64.org List-ID: On Tue, 2007-10-02 at 07:15 +0200, Andi Kleen wrote: > On Mon, Oct 01, 2007 at 08:57:10PM -0600, Thayne Harbaugh wrote: > For mmap you can emulate it by passing a low hint != 0 (e.g. getpagesize()) > in address but without MAP_FIXED and checking if the result is not beyond > your range. Cool. That's a much better solution for multiple reasons - like you mention, MAP_32BIT is only 2GB as well as it's only available on x86_64. > > > Given for mremap() it is not that easy because there is no "hint" argument > > > without MREMAP_FIXED; but unless someone really needs it i would prefer > > > to not propagate the hack. If it's really needed it's probably better > > > to implement a start search hint for mremap() > > > > It came up for user-mode Qemu for the case of emulating 32bit archs on > > x86_64 using mmap. At the moment it calls mmap with MAP_32BIT and then > > That would limit the 32bit architectures to 2GB; but their real limit > is 4GB. Losing half of the address space definitely would make users unhappy > (e.g. at least normal Linux kernels wouldn't run at all) Keeping a kernel happy isn't necessary since it's user-space emulation rather than full emulation. It is, however, useful to have 4GB rather than 2GB. > Does qemu actually need mremap() ? It would surprise me because > a lot of other OS don't implement it. Qemu has two modes: full hardware emulation and user-mode emulation. User-mode emulation translates the user-mode code and then remaps the system calls directly into the native kernel (that way all the kernel and all the I/O runs natively and faster). As far as mremap(), I'm trying to get a 32bit arm mremap() emulated syscall mapped onto a 64bit x86_64 mremap(). -- 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