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
