mm/init: remove the unnecessary special treatment for memory-less node

Because memory-less node's ->node_present_pages and its zone's
->present_pages are all 0, the judgement before calling node_set_state()
to set N_MEMORY, N_HIGH_MEMORY, N_NORMAL_MEMORY for node is enough to skip
memory-less node.  The 'continue;' statement inside for_each_node() loop
of free_area_init() is gilding the lily.

Here, remove the special handling to make memory-less node share the same
code flow as normal node.

And also rephrase the code comments above the 'continue' statement
and move them above above line 'if (pgdat->node_present_pages)'.

[bhe@redhat.com: redo code comments, per Mike]
  Link: https://lkml.kernel.org/r/ZhYJAVQRYJSTKZng@MiWiFi-R3L-srv
Link: https://lkml.kernel.org/r/20240326061134.1055295-3-bhe@redhat.comSigned-off-by: Baoquan He <bhe@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Mike Rapoport (IBM)" <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mm_init.c

@@ -1835,28 +1835,23 @@ void __init free_area_init(unsigned long *max_zone_pfn)
 				panic("Cannot allocate %zuB for node %d.\n",
 				       sizeof(*pgdat), nid);
 			arch_refresh_nodedata(nid, pgdat);
-			free_area_init_node(nid);
-
-			/*
-			 * We do not want to confuse userspace by sysfs
-			 * files/directories for node without any memory
-			 * attached to it, so this node is not marked as
-			 * N_MEMORY and not marked online so that no sysfs
-			 * hierarchy will be created via register_one_node for
-			 * it. The pgdat will get fully initialized by
-			 * hotadd_init_pgdat() when memory is hotplugged into
-			 * this node.
-			 */
-			continue;
 		}
 
 		pgdat = NODE_DATA(nid);
 		free_area_init_node(nid);
 
-		/* Any memory on that node */
-		if (pgdat->node_present_pages)
+		/*
+		 * No sysfs hierarcy will be created via register_one_node()
+		 *for memory-less node because here it's not marked as N_MEMORY
+		 *and won't be set online later. The benefit is userspace
+		 *program won't be confused by sysfs files/directories of
+		 *memory-less node. The pgdat will get fully initialized by
+		 *hotadd_init_pgdat() when memory is hotplugged into this node.
+		 */
+		if (pgdat->node_present_pages) {
 			node_set_state(nid, N_MEMORY);
-		check_for_memory(pgdat);
+			check_for_memory(pgdat);
+		}
 	}
 
 	calc_nr_kernel_pages();