linux-kernel - Re: [PATCH v5] add MAP

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Date:	Thu, 14 Jan 2010 16:30:51 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH v5] add MAP_UNLOCKED mmap flag

> On Thu, Jan 14, 2010 at 04:02:42PM +0900, KOSAKI Motohiro wrote:
> > > On Thu, Jan 14, 2010 at 09:31:03AM +0900, KOSAKI Motohiro wrote:
> > > > > 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
> > > > > in a thread safe manner. 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>
> > > > > ---
> > > > > 
> > > > > I get reports that people find this useful, so resending.
> > > > 
> > > > This description is still wrong. It doesn't describe why this patch is useful.
> > > > 
> > > I think the text above describes the feature it adds and its use
> > > case quite well. Can you elaborate what is missing in your opinion,
> > > or suggest alternative text please?
> > 
> > My point is, introducing mmap new flags need strong and clearly use-case.
> > All patch should have good benefit/cost balance. the code can describe the cost,
> > but the benefit can be only explained by the patch description.
> > 
> > I don't think this poor description explained bit benefit rather than cost.
> > you should explain why this patch is useful and not just pretty toy.
> > 
> The benefit is that with this patch I can lock all of my application in
> memory except some very big memory areas. My use case is that I want to
> run virtual machine in such a way that everything related to machine
> emulator is locked into the memory, but guest address space can be
> swapped out at will. Guest address space is so huge that it is not
> possible to allocated it locked and then unlock. I was very surprised
> that current Linux API has no way to do it hence this patch. It may look
> like a pretty toy to you until some day you need this and has no way to
> do it.

Hmm..
Your answer didn't match I wanted.
few additional questions.

- Why don't you change your application? It seems natural way than kernel change.
- Why do you want your virtual machine have mlockall? AFAIK, current majority
  virtual machine doesn't.
- If this feature added, average distro user can get any benefit?

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.



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