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

[Sumanth Korikkar]

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().

Moreover, the introduction of the 'memmap on memory' feature on s390
brings a new physical memory notifier, and functions like __add_pages()
or arch_add_memory() are consistently invoked with the mem_hotplug_lock
already acquired.

Considering these factors, it seemed more natural to move
mem_hotplug_lock in memory_block_online() and memory_block_offline(),
which was described as "fixing locking order" in the subject. 
I will change the subject to "add missing locking", if it is misleading .

Would you or Oscar agree that there is a need for those
mhp_(de)init_memmap_on_memory() functions to take lock at all?

Thanks