On Thu, Aug 19, 2021 at 03:59:56PM -0400, Felix Kuehling wrote:

I got lost trying to understand how DAX counts page references and how
the PTE_SPECIAL option affects that. Theodore, can you help with this?
Is there an easy way to test without CONFIG_ARCH_HAS_PTE_SPECIAL on x86,
or do we need to test on a CPU architecture that doesn't support this
feature?

I think the right answer is to simplify disallow ZONE_DEVICE pages
if ARCH_HAS_PTE_SPECIAL is not supported.  ARCH_HAS_PTE_SPECIAL is
supported by all modern architecture ports than can make use of
ZONE_DEVICE / dev_pagemap, so we can avoid this pocket of barely
testable code entirely:

diff --git a/mm/Kconfig b/mm/Kconfig
index 40a9bfcd5062e1..2823bbfd1c8c70 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -775,6 +775,7 @@ config ZONE_DMA32
 
 config ZONE_DEVICE
 	bool "Device memory (pmem, HMM, etc...) hotplug support"
+	depends on ARCH_HAS_PTE_SPECIAL
 	depends on MEMORY_HOTPLUG
 	depends on MEMORY_HOTREMOVE
 	depends on SPARSEMEM_VMEMMAP