mm/sparsemem: get address to page struct instead of address to pfn

memmap should be the address to page struct instead of address to pfn.

As mentioned by David, if system memory and devmem sit within a section,
the mismatch address would lead kdump to dump unexpected memory.

Since sub-section only works for SPARSEMEM_VMEMMAP, pfn_to_page() is valid
to get the page struct address at this point.

Fixes: ba72b4c8 ("mm/sparsemem: support sub-section hotplug")
Signed-off-by: NWei Yang <richardw.yang@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Acked-by: NDavid Hildenbrand <david@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Baoquan He <bhe@redhat.com>
Link: http://lkml.kernel.org/r/20200210005048.10437-1-richardw.yang@linux.intel.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -894,7 +894,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
 
 	/* Align memmap to section boundary in the subsection case */
 	if (section_nr_to_pfn(section_nr) != start_pfn)
-		memmap = pfn_to_kaddr(section_nr_to_pfn(section_nr));
+		memmap = pfn_to_page(section_nr_to_pfn(section_nr));
 	sparse_init_one_section(ms, section_nr, memmap, ms->usage, 0);
 
 	return 0;