Re: [PATCH -next v2] mm/hotplug: fix a null-ptr-deref during NUMA boot

[Qian Cai <cai@lca.pw>]

On Mon, 2019-05-13 at 14:41 +0200, Michal Hocko wrote:
> On Sun 12-05-19 01:48:29, Qian Cai wrote:
> > The linux-next commit ("x86, numa: always initialize all possible
> > nodes") introduced a crash below during boot for systems with a
> > memory-less node. This is due to CPUs that get onlined during SMP boot,
> > but that onlining triggers a page fault in bus_add_device() during
> > device registration:
> > 
> > 	error = sysfs_create_link(&bus->p->devices_kset->kobj,
> > 
> > bus->p is NULL. That "p" is the subsys_private struct, and it should
> > have been set in,
> > 
> > 	postcore_initcall(register_node_type);
> > 
> > but that happens in do_basic_setup() after smp_init().
> > 
> > The old code had set this node online via alloc_node_data(), so when it
> > came time to do_cpu_up() -> try_online_node(), the node was already up
> > and nothing happened.
> > 
> > Now, it attempts to online the node, which registers the node with
> > sysfs, but that can't happen before the 'node' subsystem is registered.
> > 
> > Since kernel_init() is running by a kernel thread that is in
> > SYSTEM_SCHEDULING state, fixed this by skipping registering with sysfs
> > during the early boot in __try_online_node().
> 
> Relying on SYSTEM_SCHEDULING looks really hackish. Why cannot we simply
> drop try_online_node from do_cpu_up? Your v2 remark below suggests that
> we need to call node_set_online because something later on depends on
> that. Btw. why do we even allocate a pgdat from this path? This looks
> really messy.

See the commit cf23422b9d76 ("cpu/mem hotplug: enable CPUs online before local
memory online")

It looks like try_online_node() in do_cpu_up() is needed for memory hotplug
which is to put its node online if offlined and then hotadd_new_pgdat() calls
build_all_zonelists() to initialize the zone list.

> 
> > Call Trace:
> >  device_add+0x43e/0x690
> >  device_register+0x107/0x110
> >  __register_one_node+0x72/0x150
> >  __try_online_node+0x8f/0xd0
> >  try_online_node+0x2b/0x50
> >  do_cpu_up+0x46/0xf0
> >  cpu_up+0x13/0x20
> >  smp_init+0x6e/0xd0
> >  kernel_init_freeable+0xe5/0x21f
> >  kernel_init+0xf/0x180
> >  ret_from_fork+0x1f/0x30
> > 
> > Reported-by: Barret Rhoden <brho@google.com>
> > Signed-off-by: Qian Cai <cai@lca.pw>
> > ---
> > 
> > v2: Set the node online as it have CPUs. Otherwise, those memory-less nodes
> > will
> >     end up being not in sysfs i.e., /sys/devices/system/node/.
> > 
> >  mm/memory_hotplug.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index b236069ff0d8..6eb2331fa826 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1037,6 +1037,18 @@ static int __try_online_node(int nid, u64 start, bool
> > set_node_online)
> >  	if (node_online(nid))
> >  		return 0;
> >  
> > +	/*
> > +	 * Here is called by cpu_up() to online a node without memory from
> > +	 * kernel_init() which guarantees that "set_node_online" is true
> > which
> > +	 * will set the node online as it have CPUs but not ready to call
> > +	 * register_one_node() as "node_subsys" has not been initialized
> > +	 * properly yet.
> > +	 */
> > +	if (system_state == SYSTEM_SCHEDULING) {
> > +		node_set_online(nid);
> > +		return 0;
> > +	}
> > +
> >  	pgdat = hotadd_new_pgdat(nid, start);
> >  	if (!pgdat) {
> >  		pr_err("Cannot online node %d due to NULL pgdat\n", nid);
> > -- 
> > 2.20.1 (Apple Git-117)
> 
>