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SubjectRe: [GIT PULL] core kernel fixes
On Sat, 20 Jun 2009, Linus Torvalds 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.

Hmm. The main reason why we switched to get_user_pages_fast() in the
futex code is to avoid mmap_sem contention which was observed as a
real big performance problem especially with those horrible JavaVM
applications.

As a fallout of this we got rid of the private find_vma /
handle_mm_fault magic (as above) in the futex code which mm folks
frowned upon for quite a while. Unfortunately we got it wrong :(

I agree that in the fault path we might go back to the mmap_sem
version, but I want to avoid it when possible. At least we need to run
some of those horrible JavaVM apps to see whether it matters or not.

OTOH, I really wonder whether we can simplify the whole logic when we
keep the page reference for the atomic access to the user space
address, but I still have doubts about the life time rules of all
this. Also I need to find out how this would affect the fast path
optimization of private futexes where we avoid the lookup in the first
place.

Thanks,

tglx


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