mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM

When we use SPARSEMEM instead of SPARSEMEM_VMEMMAP, pfn_to_page()
doesn't work before sparse_init_one_section() is called. This leads to a
crash when hotplug memory.

PGD 0 P4D 0
Oops: 0002 [#1] SMP PTI
CPU: 3 PID: 8 Comm: kworker/u16:0 Tainted: G        W         5.5.0-next-20200205+ torvalds#339
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
Workqueue: kacpi_hotplug acpi_hotplug_work_fn
RIP: 0010:__memset+0x24/0x30
Call Trace:
 sparse_add_section+0x150/0x1d8
 __add_pages+0xbf/0x150
 add_pages+0x12/0x60
 add_memory_resource+0xc8/0x210
 ? wake_up_q+0xa0/0xa0
 __add_memory+0x62/0xb0
 acpi_memory_device_add+0x13f/0x300
 acpi_bus_attach+0xf6/0x200
 acpi_bus_scan+0x43/0x90
 acpi_device_hotplug+0x275/0x3d0
 acpi_hotplug_work_fn+0x1a/0x30
 process_one_work+0x1a7/0x370
 worker_thread+0x30/0x380
 ? flush_rcu_work+0x30/0x30
 kthread+0x112/0x130
 ? kthread_create_on_node+0x60/0x60
 ret_from_fork+0x35/0x40

We should use memmap as it did.

Fixes: ba72b4c ("mm/sparsemem: support sub-section hotplug")
Signed-off-by: Wei Yang <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Baoquan He <[email protected]>
CC: Dan Williams <[email protected]>
Signed-off-by: Baoquan He <[email protected]>

mm/sparse.c

@@ -944,7 +944,7 @@ int __meminit sparse_add_section(int nid, unsigned long start_pfn,
 	 * Poison uninitialized struct pages in order to catch invalid flags
 	 * combinations.
 	 */
-	page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr_pages);
+	page_init_poison(memmap, sizeof(struct page) * nr_pages);
 
 	ms = __nr_to_section(section_nr);
 	set_section_nid(section_nr, nid);