[RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

[Jason Gunthorpe]

On Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:

> As Gerald mentioned, it is very difficult to explain in a clear way.
> Hopefully, one could make sense ot of it.

I would say the page table API requires this invariant:

        pud = pud_offset(p4d, addr);
        do {
		WARN_ON(pud != pud_offset(p4d, addr);
                next = pud_addr_end(addr, end);
        } while (pud++, addr = next, addr != end);

ie pud++ is supposed to be a shortcut for 
  pud_offset(p4d, next)

While S390 does not follow this. Fixing addr_end brings it into
alignment by preventing pud++ from happening.

The only currently known side effect is that gup_fast crashes, but it
sure is an unexpected thing.

This suggests another fix, which is to say that pud++ is undefined and
pud_offset() must always be called, but I think that would cause worse
codegen on all other archs.

Jason