On Wed, Aug 18, 2021 at 12:28:30PM -0700, Ralph Campbell wrote:

Did you test on a system without CONFIG_ARCH_HAS_PTE_SPECIAL defined?
In that case, mmap() of a DAX device will call insert_page() which calls
get_page() which would trigger VM_BUG_ON_PAGE().

__vm_insert_mixed still ends up calling insert_pfn for the
!CASE_ARCH_HAS_PTE_SPECIAL if pfn_t_devmap() is true, which it should
be for DAX.  (and as said in my other mail, I suspect we should disallow
that case anyway, as no one can test it in practice).