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

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

On Sun, Feb 22, 2026 at 01:21:42PM +0200, Mike Rapoport wrote:
> Hi,
> 
> On Sun, Feb 22, 2026 at 01:44:51PM +0800, Ming Lei wrote:
> > 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.
>  
> I believe that this change is fine for !x86 as well, but it deserves a
> sentence in the commit log.
> 
> > Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
> > Fixes: d49004c5f0c1 ("arch, mm: consolidate initialization of nodes, zones and memory map")
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  mm/mm_init.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > index 61d983d23f55..9d63cab36204 100644
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -1896,7 +1896,7 @@ static void __init free_area_init(void)
> >  	for_each_node(nid) {
> >  		pg_data_t *pgdat;
> >  
> > -		if (!node_online(nid))
> > +		if (!NODE_DATA(nid))
> >  			alloc_offline_node_data(nid);
> 
> A comment that says that if an architecture didn't allocate node data, we
> presume that the node is memoryless and offline would be nice here.

Hi Mike,

All are addressed in V2:

https://lore.kernel.org/linux-mm/20260222115702.3659-1-ming.lei@redhat.com/

But miss to Cc you, sorry...

Thanks,
Ming