From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752069AbcEKLcd (ORCPT ); Wed, 11 May 2016 07:32:33 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:33925 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751397AbcEKLcb (ORCPT ); Wed, 11 May 2016 07:32:31 -0400 Date: Wed, 11 May 2016 13:32:29 +0200 From: Michal Hocko To: Peter Zijlstra Cc: Michael Kerrisk , Andrew Morton , Linus Torvalds , David Rientjes , LKML , Linux API , linux-mm@kvack.org Subject: Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic Message-ID: <20160511113228.GJ16677@dhcp22.suse.cz> References: <1431527892-2996-1-git-send-email-miso@dhcp22.suse.cz> <1431527892-2996-2-git-send-email-miso@dhcp22.suse.cz> <57331275.9000805@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57331275.9000805@infradead.org> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 11-05-16 13:07:33, Peter Zijlstra wrote: > > > On 05/13/2015 04:38 PM, Michal Hocko wrote: > > From: Michal Hocko > > > > MAP_LOCKED had a subtly different semantic from mmap(2)+mlock(2) since > > it has been introduced. > > mlock(2) fails if the memory range cannot get populated to guarantee > > that no future major faults will happen on the range. mmap(MAP_LOCKED) on > > the other hand silently succeeds even if the range was populated only > > partially. > > > > Fixing this subtle difference in the kernel is rather awkward because > > the memory population happens after mm locks have been dropped and so > > the cleanup before returning failure (munlock) could operate on something > > else than the originally mapped area. > > > > E.g. speculative userspace page fault handler catching SEGV and doing > > mmap(fault_addr, MAP_FIXED|MAP_LOCKED) might discard portion of a racing > > mmap and lead to lost data. Although it is not clear whether such a > > usage would be valid, mmap page doesn't explicitly describe requirements > > for threaded applications so we cannot exclude this possibility. > > > > This patch makes the semantic of MAP_LOCKED explicit and suggest using > > mmap + mlock as the only way to guarantee no later major page faults. > > > > URGH, this really blows chunks. It basically means MAP_LOCKED is pointless > cruft and we might as well remove it. Yeah, the usefulness of MAP_LOCKED is somehow reduced. Everybody who wants the full semantic really have to use mlock(2). > Why not fix it proper? I have tried but it turned out to be a problem because we are dropping mmap_sem after we initialized VMA and as Linus pointed out there are multithreaded applications which are doing opportunistic memory management[1]. So we would have to hold the mmap_sem for write during the whole VMA setup + population and that doesn't seem to be worth all the trouble when we are even not sure whether somebody relies on MAP_LOCKED to have the hard mlock semantic. --- [1] http://lkml.kernel.org/r/CA+55aFydkG-BgZzry5DrTzueVh9VvEcVJdLV8iOyUphQk=0vpw@mail.gmail.com -- Michal Hocko SUSE Labs