drivers/base/memory: fix trying offlining memory blocks with memory holes on aarch64

offline_pages() properly checks for memory holes and bails out.
However, we do a page_zone(pfn_to_page(start_pfn)) before calling
offline_pages() when offlining a memory block.

We should not unconditionally call page_zone(pfn_to_page(start_pfn)) on
aarch64 in offlining code, otherwise we can trigger a BUG when hitting a
memory hole:

   kernel BUG at include/linux/mm.h:1383!
   Internal error: Oops - BUG: 0 [#1] SMP
   Modules linked in: loop processor efivarfs ip_tables x_tables ext4 mbcache jbd2 dm_mod igb nvme i2c_algo_bit mlx5_core i2c_core nvme_core firmware_class
   CPU: 13 PID: 1694 Comm: ranbug Not tainted 5.12.0-next-20210524+ #4
   Hardware name: MiTAC RAPTOR EV-883832-X3-0001/RAPTOR, BIOS 1.6 06/28/2020
   pstate: 60000005 (nZCv daif -PAN -UAO -TCO BTYPE=--)
   pc : memory_subsys_offline+0x1f8/0x250
   lr : memory_subsys_offline+0x1f8/0x250
   Call trace:
     memory_subsys_offline+0x1f8/0x250
     device_offline+0x154/0x1d8
     online_store+0xa4/0x118
     dev_attr_store+0x44/0x78
     sysfs_kf_write+0xe8/0x138
     kernfs_fop_write_iter+0x26c/0x3d0
     new_sync_write+0x2bc/0x4f8
     vfs_write+0x718/0xc88
     ksys_write+0xf8/0x1e0
     __arm64_sys_write+0x74/0xa8
     invoke_syscall.constprop.0+0x78/0x1e8
     do_el0_svc+0xe4/0x298
     el0_svc+0x20/0x30
     el0_sync_handler+0xb0/0xb8
     el0_sync+0x178/0x180
   Kernel panic - not syncing: Oops - BUG: Fatal exception
   SMP: stopping secondary CPUs
   Kernel Offset: disabled
   CPU features: 0x00000251,20000846
   Memory Limit: none

If nr_vmemmap_pages is set, we know that we are dealing with hotplugged
memory that doesn't have any holes.  So call
page_zone(pfn_to_page(start_pfn)) only when really necessary -- when
nr_vmemmap_pages is set and we actually adjust the present pages.

Link: https://lkml.kernel.org/r/20210526075226.5572-1-david@redhat.com
Fixes: a08a2ae3 ("mm,memory_hotplug: allocate memmap from the added memory range")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Qian Cai (QUIC) <quic_qiancai@quicinc.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/base/memory.c

@@ -218,14 +218,14 @@ static int memory_block_offline(struct memory_block *mem)
 	struct zone *zone;
 	int ret;
 
-	zone = page_zone(pfn_to_page(start_pfn));
-
 	/*
 	 * Unaccount before offlining, such that unpopulated zone and kthreads
 	 * can properly be torn down in offline_pages().
 	 */
-	if (nr_vmemmap_pages)
+	if (nr_vmemmap_pages) {
+		zone = page_zone(pfn_to_page(start_pfn));
 		adjust_present_page_count(zone, -nr_vmemmap_pages);
+	}
 
 	ret = offline_pages(start_pfn + nr_vmemmap_pages,
 			    nr_pages - nr_vmemmap_pages);