Re: [rfc] x86: optimise page fault path a little

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 13 Nov 2008, Nick Piggin wrote:
> 
> It's only about 1.1% on the profile of the workload I'm looking at, so my
> improvement is pretty close to in the noise, but I wonder if micro
> optimisations like the following would be welcome?

I think splitting it up is good, but I hate how your split-up ends up also 
splitting the locking (ie now you do a "down_read()" and "up_read()" in 
different functions).

I also think that to some degree you made it less readable, particularly 
this area:

	+       if (write) {
	+               if (unlikely(!(vma->vm_flags & VM_WRITE))) {
	+                       bad_area_accerr(regs, error_code, address);
	+                       return;
	+               }
	+       } else if (unlikely(error_code & PF_PROT)) {
	+               bad_area_accerr(regs, error_code, address);
	+               return;
	+       } else if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE)))) {
	+               bad_area_accerr(regs, error_code, address);
	+               return;

makes me go "whaa?" and I wonder if it wouldn't be nicer to have one 
complex conditional hidden in an inline function, and then just have

	if (unlikely(access_error(write, error_code, vma))) {
		bad_area_access_error(regs, error_code, address);
		return;
	}

where the point is that we don't want to duplicate the error case three 
times, and that "accerr" is bad naming.

IOW, I do think that the patch looks like a step in the right direction, 
but cleanliness should be a primary concern.

		Linus