powerpc/mm: protect linear mapping modifications by a mutex

This code currently relies on mem_hotplug_begin()/mem_hotplug_done() -
create_section_mapping()/remove_section_mapping() implementations
cannot tollerate getting called concurrently.

Let's prepare for callers (memtrace) not holding any such locks (and
don't force them to mess with memory hotplug locks).

Other parts in these functions don't seem to rely on external locking.
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111145322.15793-5-david@redhat.com

arch/powerpc/mm/mem.c

@@ -58,6 +58,7 @@
 #define CPU_FTR_NOEXECUTE	0
 #endif
 
+static DEFINE_MUTEX(linear_mapping_mutex);
 unsigned long long memory_limit;
 bool init_mem_is_free;
 
@@ -126,8 +127,10 @@ int __ref arch_create_linear_mapping(int nid, u64 start, u64 size,
 	int rc;
 
 	start = (unsigned long)__va(start);
+	mutex_lock(&linear_mapping_mutex);
 	rc = create_section_mapping(start, start + size, nid,
 				    params->pgprot);
+	mutex_unlock(&linear_mapping_mutex);
 	if (rc) {
 		pr_warn("Unable to create linear mapping for 0x%llx..0x%llx: %d\n",
 			start, start + size, rc);
@@ -144,7 +147,9 @@ void __ref arch_remove_linear_mapping(u64 start, u64 size)
 	start = (unsigned long)__va(start);
 	flush_dcache_range_chunked(start, start + size, FLUSH_CHUNK_SIZE);
 
+	mutex_lock(&linear_mapping_mutex);
 	ret = remove_section_mapping(start, start + size);
+	mutex_unlock(&linear_mapping_mutex);
 	WARN_ON_ONCE(ret);
 
 	/* Ensure all vmalloc mappings are flushed in case they also