Sure there can be, because we only hold the mm->page_table_lock for this,
somebody could be faulting through another mm sharing the page table. For
this reason I believe I have to look at the page table count, and unless
it's one, I have to do some extra exclusion.
> (NOTE! Sure, there might be another mm that has the same pmd shared, but
> that one is going to do an unshare before it actually touches anything in
> the pmd, so it's NOT going to change the values in the original pmd).
Actually, I was planning to keep the tables shared, even through swapin/
swapout. The data remains valid for all mm's, whether it's in ram or in
swap.
> So I'm personally convinced that the locking shouldn't be needed at all,
> if you just make sure that you do things in the right order (that, of
> course, might need some memory barriers, which had better be implied by
> the atomic dec-and-test anyway).
You've convinced me that it can be considerably streamlined, which is
great, but it can't all go, and even now there's some missing.
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