mm/memory_hotplug: don't free usage map when removing a re-added early section

When we remove an early section, we don't free the usage map, as the
usage maps of other sections are placed into the same page.  Once the
section is removed, it is no longer an early section (especially, the
memmap is freed).  When we re-add that section, the usage map is reused,
however, it is no longer an early section.  When removing that section
again, we try to kfree() a usage map that was allocated during early
boot - bad.

Let's check against PageReserved() to see if we are dealing with an
usage map that was allocated during boot.  We could also check against
!(PageSlab(usage_page) || PageCompound(usage_page)), but PageReserved() is
cleaner.

Can be triggered using memtrace under ppc64/powernv:

  $ mount -t debugfs none /sys/kernel/debug/
  $ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
  $ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
   ------------[ cut here ]------------
   kernel BUG at mm/slub.c:3969!
   Oops: Exception in kernel mode, sig: 5 [#1]
   LE PAGE_SIZE=3D64K MMU=3DHash SMP NR_CPUS=3D2048 NUMA PowerNV
   Modules linked in:
   CPU: 0 PID: 154 Comm: sh Not tainted 5.5.0-rc2-next-20191216-00005-g0be1dba7b7c0 #61
   NIP kfree+0x338/0x3b0
   LR section_deactivate+0x138/0x200
   Call Trace:
     section_deactivate+0x138/0x200
     __remove_pages+0x114/0x150
     arch_remove_memory+0x3c/0x160
     try_remove_memory+0x114/0x1a0
     __remove_memory+0x20/0x40
     memtrace_enable_set+0x254/0x850
     simple_attr_write+0x138/0x160
     full_proxy_write+0x8c/0x110
     __vfs_write+0x38/0x70
     vfs_write+0x11c/0x2a0
     ksys_write+0x84/0x140
     system_call+0x5c/0x68
   ---[ end trace 4b053cbd84e0db62 ]---

The first invocation will offline+remove memory blocks.  The second
invocation will first add+online them again, in order to offline+remove
them again (usually we are lucky and the exact same memory blocks will
get "reallocated").

Tested on powernv with boot memory: The usage map will not get freed.
Tested on x86-64 with DIMMs: The usage map will get freed.

Using Dynamic Memory under a Power DLAPR can trigger it easily.

Triggering removal (I assume after previously removed+re-added) of
memory from the HMC GUI can crash the kernel with the same call trace
and is fixed by this patch.

Link: http://lkml.kernel.org/r/20191217104637.5509-1-david@redhat.com
Fixes: 326e1b8f ("mm/sparsemem: introduce a SECTION_IS_EARLY flag")
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Pingfan Liu <piliu@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -777,7 +777,14 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 	if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION)) {
 		unsigned long section_nr = pfn_to_section_nr(pfn);
 
-		if (!section_is_early) {
+		/*
+		 * When removing an early section, the usage map is kept (as the
+		 * usage maps of other sections fall into the same page). It
+		 * will be re-used when re-adding the section - which is then no
+		 * longer an early section. If the usage map is PageReserved, it
+		 * was allocated during boot.
+		 */
+		if (!PageReserved(virt_to_page(ms->usage))) {
 			kfree(ms->usage);
 			ms->usage = NULL;
 		}