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

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On 15.11.23 14:41, Sumanth Korikkar wrote:
On Tue, Nov 14, 2023 at 07:22:33PM +0100, David Hildenbrand wrote:
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

mhp_(de)init_memmap_on_memory() functions can change zone stats and
struct page content, but they are currently called w/o the

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?

At present, the functions online_pages() and offline_pages() acquire the
mem_hotplug_lock right at the start. However, given the necessity of
locking in mhp_(de)init_memmap_on_memory(), it would be more efficient
to consolidate the locking process by holding the mem_hotplug_lock once
in memory_block_online() and memory_block_offline().

Good point; can you similarly add comments to these two functions that they need that lock in write mode?


David / dhildenb

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