linux-kernel - Re: [GIT PULL] core kernel fixes

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Date:	Sat, 20 Jun 2009 12:01:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] core kernel fixes



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.

		Linus
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