[PATCH resend] powerpc/64s: fix page table fragment refcount race vs speculative references

[Matthew Wilcox]

On Fri, Jul 27, 2018 at 09:48:17PM +1000, Nicholas Piggin wrote:
> The page table fragment allocator uses the main page refcount racily
> with respect to speculative references. A customer observed a BUG due
> to page table page refcount underflow in the fragment allocator. This
> can be caused by the fragment allocator set_page_count stomping on a
> speculative reference, and then the speculative failure handler
> decrements the new reference, and the underflow eventually pops when
> the page tables are freed.

Oof.  Can't you fix this instead by using page_ref_add() instead of
set_page_count()?

> Any objection to the struct page change to grab the arch specific
> page table page word for powerpc to use? If not, then this should
> go via powerpc tree because it's inconsequential for core mm.

I want (eventually) to get to the point where every struct page carries
a pointer to the struct mm that it belongs to.  It's good for debugging
as well as handling memory errors in page tables.