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

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Date:   Thu, 16 Nov 2023 19:40:34 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Sumanth Korikkar <sumanthk@...ux.ibm.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] mm/memory_hotplug: fix memory hotplug locking order

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