Re: [PATCH 0/2] mm/memory_hotplug: fixup crash during uevent handling

[David Hildenbrand <david@redhat.com>]

On 01.07.25 13:41, Hannes Reinecke wrote:
> From: Hannes Reinecke <hare@suse.de>
> 
> Hi all,

Hi,

> 
> we have some udev rules trying to read the sysfs attribute 'valid_zones' during
> an memory 'add' event, causing a crash in zone_for_pfn_range(). Debugging found
> that mem->nid was set to NUMA_NO_NODE, which crashed in NODE_DATA(nid).
> Further analysis revealed that we're running into a race with udev event
> processing: add_memory_resource() has this function calls:
> 
> 1) __try_online_node()
> 2) arch_add_memory()
> 3) create_memory_block_devices()
>    -> calls device_register() -> memory 'add' event
> 4) node_set_online()/__register_one_node()
>    -> calls device_register() -> node 'add' event
> 5) register_memory_blocks_under_node()
>    -> sets mem->nid
> 
> Which, to the uninitated, is ... weird ...
> 
> Why do we try to online the node in 1), but only register
> the node in 4) _after_ we have created the memory blocks in 3) ?
> And why do we set the 'nid' value in 5), when the uevent
> (which might need to see the correct 'nid' value) is sent out
> in 3) ?
> There must be a reason, I'm sure ...

Probably just they way things developed historically.

We have some weird things to handle when we have early memory blocks 
that span multiple nodes. memory_block_add_nid() is all about that, and
maintaining mem->zone accordingly.

Probably should just be cleaned up.

-- 
Cheers,

David / dhildenb