mm/memory_hotplug: export mhp_supports_memmap_on_memory()

In preparation for adding sysfs ABI to toggle memmap_on_memory semantics
for drivers adding memory, export the mhp_supports_memmap_on_memory()
helper. This allows drivers to check if memmap_on_memory support is
available before trying to request it, and display an appropriate
message if it isn't available. As part of this, remove the size argument
to this - with recent updates to allow memmap_on_memory for larger
ranges, and the internal splitting of altmaps into respective memory
blocks, the size argument is meaningless.

[akpm@linux-foundation.org: fix build]
Link: https://lkml.kernel.org/r/20240124-vv-dax_abi-v7-4-20d16cb8d23d@intel.com
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Li Zhijian <lizhijian@fujitsu.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: stellarhopper

include/linux/memory_hotplug.h

@@ -137,6 +137,7 @@ struct mhp_params {
 
 bool mhp_range_allowed(u64 start, u64 size, bool need_mapping);
 struct range mhp_get_pluggable_range(bool need_mapping);
+bool mhp_supports_memmap_on_memory(void);
 
 /*
  * Zone resizing functions
@@ -278,6 +279,11 @@ static inline bool movable_node_is_enabled(void)
 	return false;
 }
 
+static inline bool mhp_supports_memmap_on_memory(void)
+{
+	return false;
+}
+
 static inline void pgdat_kswapd_lock(pg_data_t *pgdat) {}
 static inline void pgdat_kswapd_unlock(pg_data_t *pgdat) {}
 static inline void pgdat_kswapd_lock_init(pg_data_t *pgdat) {}

mm/memory_hotplug.c

@@ -1337,7 +1337,7 @@ static inline bool arch_supports_memmap_on_memory(unsigned long vmemmap_size)
 }
 #endif
 
-static bool mhp_supports_memmap_on_memory(unsigned long size)
+bool mhp_supports_memmap_on_memory(void)
 {
 	unsigned long vmemmap_size = memory_block_memmap_size();
 	unsigned long memmap_pages = memory_block_memmap_on_memory_pages();
@@ -1346,17 +1346,11 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
 	 * Besides having arch support and the feature enabled at runtime, we
 	 * need a few more assumptions to hold true:
 	 *
-	 * a) We span a single memory block: memory onlining/offlinin;g happens
-	 *    in memory block granularity. We don't want the vmemmap of online
-	 *    memory blocks to reside on offline memory blocks. In the future,
-	 *    we might want to support variable-sized memory blocks to make the
-	 *    feature more versatile.
-	 *
-	 * b) The vmemmap pages span complete PMDs: We don't want vmemmap code
+	 * a) The vmemmap pages span complete PMDs: We don't want vmemmap code
 	 *    to populate memory from the altmap for unrelated parts (i.e.,
 	 *    other memory blocks)
 	 *
-	 * c) The vmemmap pages (and thereby the pages that will be exposed to
+	 * b) The vmemmap pages (and thereby the pages that will be exposed to
 	 *    the buddy) have to cover full pageblocks: memory onlining/offlining
 	 *    code requires applicable ranges to be page-aligned, for example, to
 	 *    set the migratetypes properly.
@@ -1368,7 +1362,7 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
 	 *       altmap as an alternative source of memory, and we do not exactly
 	 *       populate a single PMD.
 	 */
-	if (!mhp_memmap_on_memory() || size != memory_block_size_bytes())
+	if (!mhp_memmap_on_memory())
 		return false;
 
 	/*
@@ -1391,6 +1385,7 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
 
 	return arch_supports_memmap_on_memory(vmemmap_size);
 }
+EXPORT_SYMBOL_GPL(mhp_supports_memmap_on_memory);
 
 static void __ref remove_memory_blocks_and_altmaps(u64 start, u64 size)
 {
@@ -1526,7 +1521,7 @@ int __ref add_memory_resource(int nid, struct resource *res, mhp_t mhp_flags)
 	 * Self hosted memmap array
 	 */
 	if ((mhp_flags & MHP_MEMMAP_ON_MEMORY) &&
-	    mhp_supports_memmap_on_memory(memory_block_size_bytes())) {
+	    mhp_supports_memmap_on_memory()) {
 		ret = create_altmaps_and_memory_blocks(nid, group, start, size, mhp_flags);
 		if (ret)
 			goto error;