Re: [RFC] Page table sharing

[Daniel Phillips <phillips@bonn-fries.net>]

On February 19, 2002 07:11 pm, Hugh Dickins wrote:
> On Tue, 19 Feb 2002, Linus Torvalds wrote:
> > On Tue, 19 Feb 2002, Daniel Phillips wrote:
> > > >
> > > > At that point you might as well make the TLB shootdown global (ie you keep
> > > > track of a mask of CPU's whose TLB's you want to kill, and any pmd that
> > > > has count > 1 just makes that mask be "all CPU's").
> > >
> > > How do we know when to do the global tlb flush?
> > 
> > See above.
> > 
> > Basically, the algorithm is:
> > 
> > 	invalidate_cpu_mask = 0;
> > 
> > 	.. for each page swapped out ..
> > 
> > 		pte = ptep_get_and_clear(ptep);
> > 		save_pte_and_mm(pte_page(pte));
> > 		mask = mm->cpu_vm_mask;
> > 		if (page_count(pmd_page) > 1)
> > 			mask = ~0UL;
> > 		invalidate_cpu_mask |= mask;
> > 
> > and then at the end you just do
> > 
> > 	flush_tlb_cpus(invalidate_cpu_mask);
> > 	for_each_page_saved() {
> > 		free_page(page);
> > 	}
> > 
> > (yeah, yeah, add cache coherency etc).
> 
> It's a little worse than this, I think.  Propagating pte_dirty(pte) to
> set_page_dirty(page) cannot be done until after the flush_tlb_cpus,

You mean, because somebody might re-dirty an already cleaned page?  Or are
you driving at something more subtle?

> if the ptes are writable: and copy_page_range is not setting "cow", so not
> write protecting, when it's a shared writable mapping.  Easy answer is
> to scrap "cow" there and always do the write protection; but I doubt
> that's the correct answer.

Nope.  For shared mmaps you'd get tons of unecessary faults.

> swap_out could keep an array of pointers to
> ptes, to propagate dirty after flushing TLB and before freeing pages,
> but that's not very pretty.

It's not horrible, not worse than the already-existing tlb_remove_page
code anyway.  I think we're not stopped here, just slowed down for some
head scratching.

-- 
Daniel
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