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
