From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.org!.POSTED!not-for-mail From: Eli Zaretskii Newsgroups: gmane.emacs.devel Subject: Re: When should ralloc.c be used? Date: Fri, 28 Oct 2016 11:27:44 +0300 Message-ID: <83d1ikswsf.fsf@gnu.org> References: <838ttfnmev.fsf@gnu.org> <837f8znk8f.fsf@gnu.org> <83zilvm2ud.fsf@gnu.org> <83r377m0i8.fsf@gnu.org> <83eg36n6v5.fsf@gnu.org> <83shrl523p.fsf@gnu.org> <83eg354ux3.fsf@gnu.org> <4f0c2868-d408-a5c4-d5a8-90dae750eb33@dancol.org> <878tt9ggdk.fsf@ritchie.wxcvbn.org> <83k2cssypt.fsf@gnu.org> <6350b2df-fde9-e716-d279-9f29438f8ee5@dancol.org> Reply-To: Eli Zaretskii NNTP-Posting-Host: blaine.gmane.org X-Trace: blaine.gmane.org 1477643300 30993 195.159.176.226 (28 Oct 2016 08:28:20 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 28 Oct 2016 08:28:20 +0000 (UTC) Cc: eggert@cs.ucla.edu, monnier@iro.umontreal.ca, emacs-devel@gnu.org To: Daniel Colascione Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Fri Oct 28 10:28:15 2016 Return-path: Envelope-to: ged-emacs-devel@m.gmane.org Original-Received: from lists.gnu.org ([208.118.235.17]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c02Vs-00046g-Gv for ged-emacs-devel@m.gmane.org; Fri, 28 Oct 2016 10:27:44 +0200 Original-Received: from localhost ([::1]:47429 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c02Vv-0001mG-30 for ged-emacs-devel@m.gmane.org; Fri, 28 Oct 2016 04:27:47 -0400 Original-Received: from eggs.gnu.org ([2001:4830:134:3::10]:34250) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c02Vp-0001mA-PB for emacs-devel@gnu.org; Fri, 28 Oct 2016 04:27:42 -0400 Original-Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1c02Vn-0002Qd-28 for emacs-devel@gnu.org; Fri, 28 Oct 2016 04:27:41 -0400 Original-Received: from fencepost.gnu.org ([2001:4830:134:3::e]:46051) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1c02Vm-0002QY-Vp; Fri, 28 Oct 2016 04:27:39 -0400 Original-Received: from 84.94.185.246.cable.012.net.il ([84.94.185.246]:3850 helo=home-c4e4a596f7) by fencepost.gnu.org with esmtpsa (TLS1.2:RSA_AES_128_CBC_SHA1:128) (Exim 4.82) (envelope-from ) id 1c02Vm-0007lu-2q; Fri, 28 Oct 2016 04:27:38 -0400 In-reply-to: <6350b2df-fde9-e716-d279-9f29438f8ee5@dancol.org> (message from Daniel Colascione on Fri, 28 Oct 2016 01:11:08 -0700) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 2001:4830:134:3::e X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane.org@gnu.org Original-Sender: "Emacs-devel" Xref: news.gmane.org gmane.emacs.devel:208931 Archived-At: > Cc: monnier@iro.umontreal.ca, eggert@cs.ucla.edu, emacs-devel@gnu.org > From: Daniel Colascione > Date: Fri, 28 Oct 2016 01:11:08 -0700 > > Say I mmap (anonymously, for simplicity) a page PROT_NONE. After the > initial mapping, that address space is unavailable for other uses. But > because the page protections are PROT_NONE, my program has no legal > right to access that page, so the OS doesn't have to guarantee that it > can find a physical page to back that page I've mmaped. In this state, > the memory is reserved. > > The 20GB PROT_NONE address space reservation itself requires very little > memory. It's just a note in the kernel's VM interval tree that says "the > addresses in range [0x20000, 0x500020000) are reserved". Virtual memory is > > Now imagine I change the protections to PROT_READ|PROT_WRITE --- once > the PROT_READ|PROT_WRITE mprotect succeeds, my program has every right > to access that page; under a strict accounting scheme (that is, without > overcommit), the OS has to guarantee that it'll be able to go find a > physical page to back that virtual page. In this state, the memory is > committed -- the kernel has committed to finding backing storage for > that page at some point when the current process tries to access it. I'm with you up to here. My question is whether PROT_READ|PROT_WRITE call could fail after PROT_NONE succeeded. You seem to say it could; I thought it couldn't. > Say you have a strict-accounting system with 1GB of RAM and 1GB of swap. > I can write a program that reserves 20GB of address space. I thought such a reservation should fail, because you don't have enough virtual memory for 20GB of addresses. IOW, I thought the ability to reserve address space is restricted by the actual amount of virtual memory available on the system at the time of the call. You seem to say I was wrong.