linux-kernel - Re: [PATCH 1/3] mm/memory_hotplug: add missing mem_hotplug

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Date:   Mon, 20 Nov 2023 09:56:18 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 1/3] mm/memory_hotplug: add missing mem_hotplug_lock

On 20.11.23 08:23, Sumanth Korikkar wrote:
>  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.
> 
> online/offline_pages() are currently only called from
> memory_block_online/offline(), so it is safe to move the locking there.
> 
> Fixes: a08a2ae34613 ("mm,memory_hotplug: allocate memmap from the added memory range")
> Reviewed-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> Signed-off-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb