Re: [PATCH 1/8] mm/memory_hotplug: fix memory hotplug locking order

[David Hildenbrand]

On 14.11.23 19:02, Sumanth Korikkar wrote:

The patch subject talks about "fixing locking order", but it's actually 
missing locking, no?

>  From Documentation/core-api/memory-hotplug.rst:
> When adding/removing/onlining/offlining memory or adding/removing
> heterogeneous/device memory, we should always hold the mem_hotplug_lock
> in write mode to serialise memory hotplug (e.g. access to global/zone
> variables).
>
> mhp_(de)init_memmap_on_memory() functions can change zone stats and
> struct page content, but they are currently called w/o the
> mem_hotplug_lock.
>
> When memory block is being offlined and when kmemleak goes through each
> populated zone, the following theoretical race conditions could occur:
> CPU 0:					     | CPU 1:
> memory_offline()			     |
> -> offline_pages()			     |
> 	-> mem_hotplug_begin()		     |
> 	   ...				     |
> 	-> mem_hotplug_done()		     |
> 					     | kmemleak_scan()
> 					     | -> get_online_mems()
> 					     |    ...
> -> mhp_deinit_memmap_on_memory()	     |
>    [not protected by mem_hotplug_begin/done()]|
>    Marks memory section as offline,	     |   Retrieves zone_start_pfn
>    poisons vmemmap struct pages and updates   |   and struct page members.
>    the zone related data			     |
>     					     |    ...
>     					     | -> put_online_mems()
>
> Fix this by ensuring mem_hotplug_lock is taken before performing
> mhp_init_memmap_on_memory(). Also ensure that
> mhp_deinit_memmap_on_memory() holds the lock.

What speaks against grabbing that lock in these functions?

--
Cheers,

David / dhildenb