mm/ptdump: take the memory hotplug lock inside ptdump_walk_pgd()

Memory hot remove unmaps and tears down various kernel page table regions
as required.  The ptdump code can race with concurrent modifications of
the kernel page tables.  When leaf entries are modified concurrently, the
dump code may log stale or inconsistent information for a VA range, but
this is otherwise not harmful.

But when intermediate levels of kernel page table are freed, the dump code
will continue to use memory that has been freed and potentially
reallocated for another purpose.  In such cases, the ptdump code may
dereference bogus addresses, leading to a number of potential problems.

To avoid the above mentioned race condition, platforms such as arm64,
riscv and s390 take memory hotplug lock, while dumping kernel page table
via the sysfs interface /sys/kernel/debug/kernel_page_tables.

Similar race condition exists while checking for pages that might have
been marked W+X via /sys/kernel/debug/kernel_page_tables/check_wx_pages
which in turn calls ptdump_check_wx().  Instead of solving this race
condition again, let's just move the memory hotplug lock inside generic
ptdump_check_wx() which will benefit both the scenarios.

Drop get_online_mems() and put_online_mems() combination from all existing
platform ptdump code paths.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: bbd6ec605c0f ("arm64/mm: Enable memory hot remove")
Signed-off-by: Anshuman Khandual <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Dev Jain <[email protected]>
Acked-by: Alexander Gordeev <[email protected]>	[s390]
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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