Re: page table lock patch V15 [0/7]: overview

[Andi Kleen]

On Fri, Jan 14, 2005 at 03:54:59PM +1100, Nick Piggin wrote:
> On Fri, 2005-01-14 at 05:39 +0100, Andi Kleen wrote:
> 
> > As you can see cmpxchg is slightly faster for the cache hot case,
> > but incredibly slow for cache cold (probably because it does something
> > nasty on the bus). This is pretty consistent to Intel and AMD CPUs.
> > Given that page tables are likely more often cache cold than hot 
> > I would use the lazy variant. 
> > 
> 
> I have a question about your trickery with the read_pte function ;)
> 
> pte_t read_pte(volatile pte_t *pte)
> {
> 	pte_t n;
> 	do {
> 		n.pte_low = pte->pte_low;
> 		rmb();
> 		n.pte_high = pte->pte_high;
> 		rmb();
> 	} while (n.pte_low != pte->pte_low);
> 	return pte;
> }
> 
> Versus the existing set_pte function. Presumably the order here
> can't be changed otherwise you could set the present bit before
> the high bit, and race with the hardware MMU?

The hardware MMU only ever adds some bits (D etc.). Never changes
the address. It won't clear P bits.  The page fault handler also doesn't 
clear them, only the swapper does. With that knowledge you could probably 
do some optimizations.


> So I think you can get a non atomic result. Are you relying on
> assumptions about the value of pte_low not causing any problems
> in the page fault handler?

I don't know. You have to ask Christopher L. I only commented
on one subthread where he asked about atomic pte reading, 
but haven't studied his patches in detail.

-Andi
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