[PATCH V2] mm: fix NULL NODE_DATA dereference for memoryless nodes on boot

[Ming Lei <ming.lei@redhat.com>]

Commit d49004c5f0c1 ("arch, mm: consolidate initialization of nodes,
zones and memory map") moved free_area_init() from setup_arch() to
mm_core_init_early(), which runs after setup_arch() returns.

This changed the ordering relative to init_cpu_to_node() on x86. Before
the commit, free_area_init() ran during paging_init() (called from
setup_arch()) *before* init_cpu_to_node(). After the commit, it runs
*after* init_cpu_to_node().

On machines with memoryless NUMA nodes (e.g., node 0 has CPUs but no
memory), this causes a NULL pointer dereference:

 1. numa_register_nodes() skips memoryless nodes: no alloc_node_data()
    and no node_set_online() for them.
 2. init_cpu_to_node() sets memoryless nodes online (they have CPUs)
    but does not allocate NODE_DATA.
 3. free_area_init() checks "if (!node_online(nid))" to decide whether
    to call alloc_offline_node_data(). Since the memoryless node is now
    online, the allocation is skipped, leaving NODE_DATA(nid) == NULL.
 4. The immediate "pgdat = NODE_DATA(nid)" dereferences NULL.

The crash happens before console_init(), so no output is visible without
earlyprintk. With earlyprintk enabled, the following panic is observed:

 BUG: unable to handle page fault for address: 000000000002a1e0
 Oops: Oops: 0000 [#1] SMP NOPTI
 RIP: 0010:free_area_init_node+0x3a/0x540
 Call Trace:
  <TASK>
  free_area_init+0x331/0x4e0
  start_kernel+0x69/0x4a0
  x86_64_start_reservations+0x24/0x30
  x86_64_start_kernel+0x125/0x130
  common_startup_64+0x13e/0x148
  </TASK>
 Kernel panic - not syncing: Attempted to kill the idle task!

Fix this by checking "if (!NODE_DATA(nid))" instead of
"if (!node_online(nid))". This directly tests whether the per-node data
structure needs to be allocated, regardless of the node's online status.
This change is also safe for non-x86 architectures as they all allocate
NODE_DATA for every node including memoryless ones, so the check simply
evaluates to false with no change in behavior.

Fixes: d49004c5f0c1 ("arch, mm: consolidate initialization of nodes, zones and memory map")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V2:
	- add commit log for non-x86 arch
	- add comment for code change

 mm/mm_init.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 61d983d23f55..df34797691bd 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1896,7 +1896,11 @@ static void __init free_area_init(void)
 	for_each_node(nid) {
 		pg_data_t *pgdat;
 
-		if (!node_online(nid))
+		/*
+		 * If an architecture has not allocated node data for
+		 * this node, presume the node is memoryless or offline.
+		 */
+		if (!NODE_DATA(nid))
 			alloc_offline_node_data(nid);
 
 		pgdat = NODE_DATA(nid);
-- 
2.53.0