Re: [GIT PULL] core kernel fixes

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
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
Date: Sat, 20 Jun 2009 12:01:22 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LFD.2.01.0906201154230.4160@localhost.localdomain> (raw)
In-Reply-To: <alpine.LFD.2.01.0906201132020.4160@localhost.localdomain>

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