Re: [PATCH] mm: Check if PTE is already allocated during page fault

[Andrea Arcangeli <aarcange@redhat.com>]

On Fri, Apr 15, 2011 at 04:39:16PM +0200, Andrea Arcangeli wrote:
> On Fri, Apr 15, 2011 at 11:12:48AM +0100, Mel Gorman wrote:
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 5823698..1659574 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> >  	 * run pte_offset_map on the pmd, if an huge pmd could
> >  	 * materialize from under us from a different thread.
> >  	 */
> > -	if (unlikely(__pte_alloc(mm, vma, pmd, address)))
> > +	if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address))

I started hacking on this and I noticed it'd be better to extend the
unlikely through the end. At first review I didn't notice the
parenthesis closure stops after pte_none and __pte_alloc is now
uncovered. I'd prefer this:

    if (unlikely(pmd_none(*pmd) && __pte_alloc(mm, vma, pmd, address)))

I mean the real unlikely thing is that we return VM_FAULT_OOM, if we
end up calling __pte_alloc or not, depends on the app. Generally it
sounds more frequent that the pte is not none, so it's not wrong, but
it's even less likely that __pte_alloc fails so that can be taken into
account too, and __pte_alloc runs still quite frequently. So either
above or:

    if (unlikely(pmd_none(*pmd)) && unlikely(__pte_alloc(mm, vma, pmd, address)))

I generally prefer unlikely only when it's 100% sure thing it's less
likely (like the VM_FAULT_OOM), so the first version I guess it's
enough (I'm afraid unlikely for pte_none too, may make gcc generate a
far away jump possibly going out of l1 icache for a case that is only
512 times less likely at best). My point is that it's certainly hugely
more unlikely that __pte_alloc fails than the pte is none.

This is a real nitpick though ;).