mm: ZONE_DEVICE for "device memory"

While pmem is usable as a block device or via DAX mappings to userspace
there are several usage scenarios that can not target pmem due to its
lack of struct page coverage. In preparation for "hot plugging" pmem
into the vmemmap add ZONE_DEVICE as a new zone to tag these pages
separately from the ones that are subject to standard page allocations.
Importantly "device memory" can be removed at will by userspace
unbinding the driver of the device.

Having a separate zone prevents allocation and otherwise marks these
pages that are distinct from typical uniform memory.  Device memory has
different lifetime and performance characteristics than RAM.  However,
since we have run out of ZONES_SHIFT bits this functionality currently
depends on sacrificing ZONE_DMA.

Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Jerome Glisse <j.glisse@gmail.com>
[hch: various simplifications in the arch interface]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/mm/Kconfig b/mm/Kconfig
index e79de2bd12cd..a0cd086df16b 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -654,3 +654,20 @@ config DEFERRED_STRUCT_PAGE_INIT
           when kswapd starts. This has a potential performance impact on
           processes running early in the lifetime of the systemm until kswapd
           finishes the initialisation.
+
+config ZONE_DEVICE
+        bool "Device memory (pmem, etc...) hotplug support" if EXPERT
+        default !ZONE_DMA
+        depends on !ZONE_DMA
+        depends on MEMORY_HOTPLUG
+        depends on MEMORY_HOTREMOVE
+        depends on X86_64 #arch_add_memory() comprehends device memory
+
+        help
+          Device memory hotplug support allows for establishing pmem,
+          or other device driver discovered memory regions, in the
+          memmap. This allows pfn_to_page() lookups of otherwise
+          "device-physical" addresses which is needed for using a DAX
+          mapping in an O_DIRECT operation, among other things.
+
+          If FS_DAX is enabled, then say Y.