On Wed, Oct 31, 2018 at 10:08 AM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > Hi mm folks, > > I'm looking at ZONE_DEVICE support for arm64, and trying to make sense > of a build failure has led me down the rabbit hole of pfn_t.h, and > specifically __HAVE_ARCH_PTE_DEVMAP in this first instance. > > The failure itself is a link error in remove_migration_pte() due to a > missing definition of pte_mkdevmap(), but I'm a little confused at the > fact that it's explicitly declared without a definition, as if that > breakage is deliberate. It's deliberate, it's only there to allow mm/memory.c to compile. The compiler can see that pfn_t_devmap(pfn) is always false in the !__HAVE_ARCH_PTE_DEVMAP case and throws away the attempt to link to pte_devmap(). The summary is that an architecture needs to devote a free/software pte bit for Linux to indicate "device pfns".
