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

David Hildenbrand
2023-11-16T18:40:43Z

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
>>> 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?
>>
> 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?

--
Cheers,

David / dhildenb