Re: [RFC] Page table sharing

[Hugh Dickins <hugh@veritas.com>]

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, 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.  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.

Hugh

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