Re: [PATCH v2 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions

[Oscar Salvador]

On Tue, Jan 12, 2021 at 01:34:53AM -0800, Dan Williams wrote:
> While pfn_to_online_page() is able to determine pfn_valid() at
> subsection granularity it is not able to reliably determine if a given
> pfn is also online if the section is mixes ZONE_{NORMAL,MOVABLE} with
> ZONE_DEVICE. This means that pfn_to_online_page() may return invalid
> @page objects. For example with a memory map like:
> 
> 100000000-1fbffffff : System RAM
>   142000000-143002e16 : Kernel code
>   143200000-143713fff : Kernel rodata
>   143800000-143b15b7f : Kernel data
>   144227000-144ffffff : Kernel bss
> 1fc000000-2fbffffff : Persistent Memory (legacy)
>   1fc000000-2fbffffff : namespace0.0
> 
> This command:
> 
> echo 0x1fc000000 > /sys/devices/system/memory/soft_offline_page
> 
> ...succeeds when it should fail. When it succeeds it touches
> an uninitialized page and may crash or cause other damage (see
> dissolve_free_huge_page()).

[...]
 
> Because the collision case is rare, and for simplicity, the
> SECTION_TAINT_ZONE_DEVICE flag is never cleared once set.
> 
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Reported-by: Michal Hocko <mhocko@xxxxxxxx>
> Reported-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

-- 
Oscar Salvador
SUSE L3