[HMM v14 06/16] 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 | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 442ac86..6e7f613 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -654,7 +654,8 @@ static void  update_end_of_memory_vars(u64 start, u64 size)
 int arch_add_memory(int nid, u64 start, u64 size, int flags)
 {
 	const int supported_flags = MEMORY_DEVICE |
-				    MEMORY_DEVICE_ALLOW_MIGRATE;
+				    MEMORY_DEVICE_ALLOW_MIGRATE |
+				    MEMORY_DEVICE_UNADDRESSABLE;
 	struct pglist_data *pgdat = NODE_DATA(nid);
 	struct zone *zone = pgdat->node_zones +
 		zone_for_memory(nid, start, size, ZONE_NORMAL,
@@ -669,7 +670,17 @@ int arch_add_memory(int nid, u64 start, u64 size, int flags)
 		return -EINVAL;
 	}
 
-	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 (!(flags & MEMORY_DEVICE_UNADDRESSABLE))
+		init_memory_mapping(start, start + size);
 
 	ret = __add_pages(nid, zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
@@ -968,7 +979,8 @@ kernel_physical_mapping_remove(unsigned long start, unsigned long end)
 int __ref arch_remove_memory(u64 start, u64 size, int flags)
 {
 	const int supported_flags = MEMORY_DEVICE |
-				    MEMORY_DEVICE_ALLOW_MIGRATE;
+				    MEMORY_DEVICE_ALLOW_MIGRATE |
+				    MEMORY_DEVICE_UNADDRESSABLE;
 	unsigned long start_pfn = start >> PAGE_SHIFT;
 	unsigned long nr_pages = size >> PAGE_SHIFT;
 	struct page *page = pfn_to_page(start_pfn);
@@ -989,7 +1001,9 @@ int __ref arch_remove_memory(u64 start, u64 size, int flags)
 	zone = page_zone(page);
 	ret = __remove_pages(zone, start_pfn, nr_pages);
 	WARN_ON_ONCE(ret);
-	kernel_physical_mapping_remove(start, start + size);
+
+	if (!(flags & MEMORY_DEVICE_UNADDRESSABLE))
+		kernel_physical_mapping_remove(start, start + size);
 
 	return ret;
 }
-- 
2.4.3