[HMM 04/16] mm/ZONE_DEVICE/x86: add support for un-addressable device memory

[JÃrÃme Glisse]

It does not need much, just skip populating kernel linear mapping
for range of un-addressable device memory (it is pick so that there
is no physical memory resource overlapping it). All the logic is in
share mm code.

Only support x86-64 as this feature doesn't make much sense with
constrained virtual address space of 32bits architecture.

Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
---
 arch/x86/mm/init_64.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 6c0b24e..b635636 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -658,6 +658,7 @@ int arch_add_memory(int nid, u64 start, u64 size, enum memory_type type)
 	case MEMORY_NORMAL:
 		break;
 	case MEMORY_DEVICE_PERSISTENT:
+	case MEMORY_DEVICE_UNADDRESSABLE:
 		for_device = true;
 		break;
 	default:
@@ -668,7 +669,17 @@ int arch_add_memory(int nid, u64 start, u64 size, enum memory_type type)
 	zone = pgdat->node_zones +
 		zone_for_memory(nid, start, size, ZONE_NORMAL, for_device);
 
-	init_memory_mapping(start, start + size);
+	/*
+	 * We get un-addressable memory when some one is adding a ZONE_DEVICE
+	 * to have struct page for a device memory which is not accessible by
+	 * the CPU so it is pointless to have a linear kernel mapping of such
+	 * memory.
+	 *
+	 * Core mm should make sure it never set a pte pointing to such fake
+	 * physical range.
+	 */
+	if (type != MEMORY_DEVICE_UNADDRESSABLE)
+		init_memory_mapping(start, start + size);
 
 	ret = __add_pages(nid, zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
@@ -980,6 +991,7 @@ int __ref arch_remove_memory(u64 start, u64 size, enum memory_type type)
 	switch (type) {
 	case MEMORY_NORMAL:
 	case MEMORY_DEVICE_PERSISTENT:
+	case MEMORY_DEVICE_UNADDRESSABLE:
 		break;
 	default:
 		pr_err("hotplug unsupported memory type %d\n", type);
@@ -993,7 +1005,9 @@ int __ref arch_remove_memory(u64 start, u64 size, enum memory_type type)
 	zone = page_zone(page);
 	ret = __remove_pages(zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
-	kernel_physical_mapping_remove(start, start + size);
+
+	if (type != MEMORY_DEVICE_UNADDRESSABLE)
+		kernel_physical_mapping_remove(start, start + size);
 
 	return ret;
 }
-- 
2.9.3