[PATCH 3/3] dax/kmem: Always enroll hotplugged memory for memmap_on_memory

[Vishal Verma <vishal.l.verma@xxxxxxxxx>]

With DAX memory regions originating from CXL memory expanders or
NVDIMMs, the kmem driver may be hot-adding huge amounts of system memory
on a system without enough 'regular' main memory to support the memmap
for it. To avoid this, ensure that all kmem managed hotplugged memory is
added with the MHP_MEMMAP_ON_MEMORY flag to place the memmap on the
new memory region being hot added.

To do this, call add_memory() in chunks of memory_block_size_bytes() as
that is a requirement for memmap_on_memory. Additionally, Use the
mhp_flag to force the memmap_on_memory checks regardless of the
respective module parameter setting.

Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Huang Ying <ying.huang@xxxxxxxxx>
Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
---
 drivers/dax/kmem.c | 49 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 36 insertions(+), 13 deletions(-)

diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index 7b36db6f1cbd..0751346193ef 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -12,6 +12,7 @@
 #include <linux/mm.h>
 #include <linux/mman.h>
 #include <linux/memory-tiers.h>
+#include <linux/memory_hotplug.h>
 #include "dax-private.h"
 #include "bus.h"
 
@@ -105,6 +106,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 	data->mgid = rc;
 
 	for (i = 0; i < dev_dax->nr_range; i++) {
+		u64 cur_start, cur_len, remaining;
 		struct resource *res;
 		struct range range;
 
@@ -137,21 +139,42 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 		res->flags = IORESOURCE_SYSTEM_RAM;
 
 		/*
-		 * Ensure that future kexec'd kernels will not treat
-		 * this as RAM automatically.
+		 * Add memory in chunks of memory_block_size_bytes() so that
+		 * it is considered for MHP_MEMMAP_ON_MEMORY
+		 * @range has already been aligned to memory_block_size_bytes(),
+		 * so the following loop will always break it down cleanly.
 		 */
-		rc = add_memory_driver_managed(data->mgid, range.start,
-				range_len(&range), kmem_name, MHP_NID_IS_MGID);
+		cur_start = range.start;
+		cur_len = memory_block_size_bytes();
+		remaining = range_len(&range);
+		while (remaining) {
+			mhp_t mhp_flags = MHP_NID_IS_MGID;
 
-		if (rc) {
-			dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
-					i, range.start, range.end);
-			remove_resource(res);
-			kfree(res);
-			data->res[i] = NULL;
-			if (mapped)
-				continue;
-			goto err_request_mem;
+			if (mhp_supports_memmap_on_memory(cur_len,
+							  MHP_MEMMAP_ON_MEMORY))
+				mhp_flags |= MHP_MEMMAP_ON_MEMORY;
+			/*
+			 * Ensure that future kexec'd kernels will not treat
+			 * this as RAM automatically.
+			 */
+			rc = add_memory_driver_managed(data->mgid, cur_start,
+						       cur_len, kmem_name,
+						       mhp_flags);
+
+			if (rc) {
+				dev_warn(dev,
+					 "mapping%d: %#llx-%#llx memory add failed\n",
+					 i, cur_start, cur_start + cur_len - 1);
+				remove_resource(res);
+				kfree(res);
+				data->res[i] = NULL;
+				if (mapped)
+					continue;
+				goto err_request_mem;
+			}
+
+			cur_start += cur_len;
+			remaining -= cur_len;
 		}
 		mapped++;
 	}

-- 
2.40.1