From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752194Ab0ARNk7 (ORCPT ); Mon, 18 Jan 2010 08:40:59 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751930Ab0ARNk6 (ORCPT ); Mon, 18 Jan 2010 08:40:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:24100 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751057Ab0ARNk5 convert rfc822-to-8bit (ORCPT ); Mon, 18 Jan 2010 08:40:57 -0500 Date: Mon, 18 Jan 2010 15:40:19 +0200 From: Gleb Natapov To: KOSAKI Motohiro Cc: "Andrew C. Morrow" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, akpm@linux-foundation.org Subject: Re: [PATCH v5] add MAP_UNLOCKED mmap flag Message-ID: <20100118134019.GH30698@redhat.com> References: <20100114170247.6747.A69D9226@jp.fujitsu.com> <6feea4871001141130j4184a24di363b7e6553d506e8@mail.gmail.com> <20100118121726.AE45.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=koi8-r Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: <20100118121726.AE45.A69D9226@jp.fujitsu.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 18, 2010 at 12:23:09PM +0900, KOSAKI Motohiro wrote: > > On Thu, Jan 14, 2010 at 3:17 AM, KOSAKI Motohiro > > wrote: > > >> > Hmm.. > > >> > Your answer didn't match I wanted. > > >> Then I don't get what you want. > > > > > > I want to know the benefit of the patch for patch reviewing. > > > > > > > The benefit of the patch is that it makes it possible for an > > application which has previously called mlockall(MCL_FUTURE) to > > selectively exempt new memory mappings from memory locking, on a > > per-mmap-call basis. As was pointed out earlier, there is currently no > > thread-safe way for an application to do this. The earlier proposed > > workaround of toggling MCL_FUTURE around calls to mmap is racy in a > > multi-threaded context. Other threads may manipulate the address space > > during the window where MCL_FUTURE is off, subverting the programmers > > intended memory locking semantics. > > > > The ability to exempt specific memory mappings from memory locking is > > necessary when the region to be mapped is larger than physical memory. > > In such cases a call to mmap the region cannot succeed, unless > > MAP_UNLOCKED is available. > > > > > > > > > >> > few additional questions. > > >> > > > >> > - Why don't you change your application? It seems natural way than kernel change. > > >> There is no way to change my application and achieve what I've described > > >> in a multithreaded app. > > > > > > Then, we don't recommend to use mlockall(). I don't hope to hear your conclusion, > > > it is not objectivization. I hope to hear why you reached such conclusion. > > > > > > > I agree that mlockall is a big hammer and should be avoided in most > > cases, but there are situations where it is exactly what is needed. In > > Gleb's instance, it sounds like he is doing some finicky performance > > measurement and major page faults skew his results. In my case, I have > > a realtime process where the measured latency impact of major page > > faults is unacceptable. In both of these cases, mlockall is a > > reasonable approach to eliminating major faults. > > > > However, Gleb and I have independently found ourselves unable to use > > mlockall because we also need to create a very large memory mapping > > (for which we don't care about major faults). The proposed > > MAP_UNLOCKED flag would allow us to override MCL_FUTURE for that one > > mapping. > > > > > > > >> > - Why do you want your virtual machine have mlockall? AFAIK, current majority > > >> > š virtual machine doesn't. > > >> It is absolutely irrelevant for that patch, but just because you ask I > > >> want to measure the cost of swapping out of a guest memory. > > > > > > No. if you stop to use mlockall, the issue is vanished. > > > > > > > And other issues arise. Gleb described a situation where the use of > > mlockall is justified, identified an issue which prevents its use, and > > provided a patch which resolves that issue. Why are you focusing on > > the validity of using mlockall? > > > > > > > >> > - If this feature added, average distro user can get any benefit? > > >> > > > >> ?! Is this some kind of new measure? There are plenty of much more > > >> invasive features that don't bring benefits to an average distro user. > > >> This feature can bring benefit to embedded/RT developers. > > > > > > I mean who get benifit? > > > > > > > > >> > I mean, many application developrs want to add their specific feature > > >> > into kernel. but if we allow it unlimitedly, major syscall become > > >> > the trushbox of pretty toy feature soon. > > >> > > > >> And if application developer wants to extend kernel in a way that it > > >> will be possible to do something that was not possible before why is > > >> this a bad thing? I would agree with you if for my problem was userspace > > >> solution, but there is none. The mmap interface is asymmetric in regards > > >> to mlock currently. There is MAP_LOCKED, but no MAP_UNLOCKED. Why > > >> MAP_LOCKED is useful then? > > > > > > Why? Because this is formal LKML reviewing process. I'm reviewing your > > > patch for YOU. > > > > > > If there is no objective reason, I don't want to continue reviewing. > > > > > > > There is an objective reason: the current interaction between > > mlockall(MCL_FUTURE) and mmap has a deficiency. In 'normal' mode, > > without MCL_FUTURE in force, the default is that new memory mappings > > are not locked, but mmap provides MAP_LOCKED specifically to override > > that default. However, with MCL_FUTURE toggled to on, there is no > > analogous way to tell mmap to override the default. The proposed > > MAP_UNLOCKED flag would resolve this deficiency. > > Very thank you, Andrew! > > Your explanation help me lots rather than original patch description. OK, At least > MAP_UNLOCED have two users (you and gleb) and your explanation seems > makes sense. > > So, if gleb resend this patch with rewrited description, I might take my reviewed-by tag to it, probagly. > Just did it. I hope the commit message is OK with you now. Its text is taken from this Andrew's mail. Thanks. -- Gleb.