linux-kernel - Re: [PATCH][RFC] add MAP

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Message-ID: <20091006121603.GK9832@redhat.com>
Date:	Tue, 6 Oct 2009 14:16:03 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH][RFC] add MAP_UNLOCKED mmap flag

On Tue, Oct 06, 2009 at 09:10:35PM +0900, KOSAKI Motohiro wrote:
> 2009/10/6 Gleb Natapov <gleb@...hat.com>:
> > On Tue, Oct 06, 2009 at 07:27:56PM +0900, KOSAKI Motohiro wrote:
> >> > On Tue, Oct 06, 2009 at 07:11:06PM +0900, KOSAKI Motohiro wrote:
> >> > > Hi
> >> > >
> >> > > > If application does mlockall(MCL_FUTURE) it is no longer possible to
> >> > > > mmap file bigger than main memory or allocate big area of anonymous
> >> > > > memory. Sometimes it is desirable to lock everything related to program
> >> > > > execution into memory, but still be able to mmap big file or allocate
> >> > > > huge amount of memory and allow OS to swap them on demand. MAP_UNLOCKED
> >> > > > allows to do that.
> >> > > >
> >> > > > Signed-off-by: Gleb Natapov <gleb@...hat.com>
> >> > >
> >> > > Why don't you use explicit munlock()?
> >> > Because mmap will fail before I'll have a chance to run munlock on it.
> >> > Actually when I run my process inside memory limited container host dies
> >> > (I suppose trashing, but haven't checked).
> >> >
> >> > > Plus, Can you please elabrate which workload nedd this feature?
> >> > >
> >> > I wanted to run kvm with qemu process locked in memory, but guest memory
> >> > unlocked. And guest memory is bigger then host memory in the case I am
> >> > testing. I found out that it is impossible currently.
> >>
> >> 1. process creation (qemu)
> >> 2. load all library
> > Can't control this if program has plugging. Not qemu case
> > though.
> >
> >> 3. mlockall(MCL_CURRENT)
> >> 4. load guest OS
> > And what about all other allocations qemu does during its life time? Not
> > all of them will be small enough to be from brk area.
> >
> >> is impossible? why?
> >>
> > Because what you are proposing is not the same as mlockall(MCL_CURRENT|MCL_FUTURE);
> >
> > You essentially say that MCL_FUTURE is not needed.
> 
> No, I only think your case doesn't fit MC_FUTURE.
> I haven't find any real benefit in this patch.
I did. It allows me to achieve something I can't now. Steps you provide
just don't fit my needs. I need all memory areas (current and feature) to be
locked except one. Very big one. You propose to lock memory at some
arbitrary point and from that point on all newly mapped memory areas will
be unlocked. Don't you see it is different?

--
			Gleb.
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