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Date: Sat, 20 Jun 2009 22:27:20 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] core kernel fixes
Message-ID: <20090620202720.GA7411@elte.hu>
References: <20090620173022.GA14145@elte.hu> <alpine.LFD.2.01.0906201132020.4160@localhost.localdomain> <alpine.LFD.2.01.0906201154230.4160@localhost.localdomain>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sat, 20 Jun 2009, Linus Torvalds wrote:
> > 
> > On x86, the natural way to do what you want done is ONE SINGLE 
> > INSTRUCTION! As far as I can tell, the above crazy function is 100% 
> > equivalent to this:
> > 
> > 	asm __inline__("lock ; addl $0,%0":"+m" (*uaddr): :"memory", "cc");
> > 
> > which really makes me think that using "get_user_pages_fast()" for it is 
> > some truly crazy crap.
> 
> We could also take the opposite approach - knowing that this is called 
> only when the page doesn't exist, and just doing
> 
> 	down_read(mmap_sem)
> 	vma = find_vma(..)
> 	ret = VM_FAULT_ERROR;
> 	if (vma && vma->vm_start <= address)
> 		ret = handle_mm_fault(mm, vma, address, 1);
> 	up_read(mmap_sem);
> 	return (ret & VM_FAULT_ERROR) ? -EFAULT : 0;
> 
> or something like that. Again, that looks saner than using 
> get_user_pages() for this and then dropping the page.

We'll sort this out tomorrow, sorry about this.

( The other embarrasing bit is that i flagged this commit as bad
  nine days ago - during review i noticed the same bad pattern of a 
  pointless get-put cycle ... but the commit stuck around and forgot 
  about it. Sloppy. )

	Ingo