Re: [PATCH v5] add MAP_UNLOCKED mmap flag

From: KOSAKI Motohiro
Date: Sun Jan 17 2010 - 22:23:29 EST

> On Thu, Jan 14, 2010 at 3:17 AM, KOSAKI Motohiro
> <kosaki.motohiro@xxxxxxxxxxxxxx> 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
> >> >   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.


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