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    SubjectRe: [PATCH v2 3/5] mm: Teach pfn_to_online_page() about ZONE_DEVICE section collisions
    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@linux-foundation.org>
    > Reported-by: Michal Hocko <mhocko@suse.com>
    > Reported-by: David Hildenbrand <david@redhat.com>
    > Signed-off-by: Dan Williams <dan.j.williams@intel.com>

    Reviewed-by: Oscar Salvador <osalvador@suse.de>

    --
    Oscar Salvador
    SUSE L3

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