Re: Q: backport of the free_pgtables tlb fixes to 2.4

[Linus Torvalds]

On Thu, 23 May 2002, Andrea Arcangeli wrote:
>
> If the userspace tlb lookup is started during munmap the tlb can contain
> garabge before invalidate_tlb.

No.

If we wait until after the TLB fill to actually free the page tables
pages, there is _no_ way the TLB can contain garbage, because the page
directories will never have had garbage in it while any TLB lookup could
be active.

Which is the whole _point_ of the patches.

> What I don't understand is how the BTB can invoke random userspace tlb
> fills when we are running do_munmap, there's no point at all in doing
> that. If the cpu see a read of an user address after invalidate_tlb,
> the tlb must not be started because it's before an invalidate_tlb.

Take a course in CPU design if you want to understand why a CPU front-end
might speculatively start accessing something before the back-end has
actually told it what the "something" actually is.

But don't argue with the patch.

> And if it's true not even irq are barriers for the tlb fills invoked by
> this p4-BTB thing

It has nothing to do with the BTB - the BTB is just a source of
speculative addresses to start looking at.

But yes, Intel tells me that the only thing that is guaranteed to
serialize a TLB lookup is a TLB invalidate. NOTHING else.

>	so if leave_mm is really necessary, then 2.5 is as
> well wrong in UP, because the pagetable can be scribbled by irqs in a UP
> machine, and so the fastmode must go away even in 1 cpu systems.

Yes. Except I will make the 2.5.x code use the pmd quicklists instead
(both fast and slow mode), since that actually ends up being "nicer" from
a cross-architecture standpoint (right now the i386 careful mode depends
on the fact that page directories are regular pages - which is not true on
other CPU's).

		Linus