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

[David Hildenbrand]

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