Re: [PATCH v5] add MAP_UNLOCKED mmap flag

Gleb Natapov
2010-01-18T13:40:59Z

On Mon, Jan 18, 2010 at 12:23:09PM +0900, KOSAKI Motohiro wrote:
> > On Thu, Jan 14, 2010 at 3:17 AM, KOSAKI Motohiro
> > <kosaki.motohiro@jp.fujitsu.com> 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.
>
Just did it. I hope the commit message is OK with you now. Its text is
taken from this Andrew's mail. Thanks.

--
Gleb.