From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id A47466B006A for ; Mon, 18 Jan 2010 08:40:25 -0500 (EST) Date: Mon, 18 Jan 2010 15:40:19 +0200 From: Gleb Natapov 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: quoted-printable In-Reply-To: <20100118121726.AE45.A69D9226@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org 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 List-ID: 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. > > > > >=20 > > 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. > >=20 > > 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. > >=20 > >=20 > > > > > >> > 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 descr= ibed > > >> 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 conclu= sion. > > > > >=20 > > 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. > >=20 > > 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. > >=20 > > > > > >> > - Why do you want your virtual machine have mlockall? AFAIK, curre= nt majority > > >> > =9A 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. > > > > >=20 > > 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? > >=20 > > > > > >> > - 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 use= r. > > >> This feature can bring benefit to embedded/RT developers. > > > > > > I mean who get benifit? > > > > > > > > >> > I mean, many application developrs want to add their specific feat= ure > > >> > 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 users= pace > > >> solution, but there is none. The mmap interface is asymmetric in reg= ards > > >> 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. > > > > >=20 > > 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. >=20 > Very thank you, Andrew! >=20 > 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. >=20 > So, if gleb resend this patch with rewrited description, I might take my = reviewed-by tag to it, probagly. >=20 Just did it. I hope the commit message is OK with you now. Its text is taken from this Andrew's mail. Thanks. -- Gleb. -- 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