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@xxxxxxxxxx>
> > Signed-off-by: Qian Cai <cai@xxxxxx>
> > ---
> >
> > 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)
>
>
