Commit 1378a5ee ("mm: store compound_nr as well as compound_order")
added compound_nr counter to first tail struct page, overlaying with
page->mapping.  The overlay itself is fine, but while freeing gigantic
hugepages via free_contig_range(), a "bad page" check will trigger for
non-NULL page->mapping on the first tail page:

  BUG: Bad page state in process bash  pfn:380001
  page:00000000c35f0856 refcount:0 mapcount:0 mapping:00000000126b68aa index:0x0 pfn:0x380001
  aops:0x0
  flags: 0x3ffff00000000000()
  raw: 3ffff00000000000 0000000000000100 0000000000000122 0000000100000000
  raw: 0000000000000000 0000000000000000 ffffffff00000000 0000000000000000
  page dumped because: non-NULL mapping
  Modules linked in:
  CPU: 6 PID: 616 Comm: bash Not tainted 5.10.0-rc7-next-20201208 #1
  Hardware name: IBM 3906 M03 703 (LPAR)
  Call Trace:
    show_stack+0x6e/0xe8
    dump_stack+0x90/0xc8
    bad_page+0xd6/0x130
    free_pcppages_bulk+0x26a/0x800
    free_unref_page+0x6e/0x90
    free_contig_range+0x94/0xe8
    update_and_free_page+0x1c4/0x2c8
    free_pool_huge_page+0x11e/0x138
    set_max_huge_pages+0x228/0x300
    nr_hugepages_store_common+0xb8/0x130
    kernfs_fop_write+0xd2/0x218
    vfs_write+0xb0/0x2b8
    ksys_write+0xac/0xe0
    system_call+0xe6/0x288
  Disabling lock debugging due to kernel taint

This is because only the compound_order is cleared in
destroy_compound_gigantic_page(), and compound_nr is set to
1U << order == 1 for order 0 in set_compound_order(page, 0).

Fix this by explicitly clearing compound_nr for first tail page after
calling set_compound_order(page, 0).

Link: https://lkml.kernel.org/r/20201208182813.66391-2-gerald.schaefer@linux.ibm.com
Fixes: 1378a5ee ("mm: store compound_nr as well as compound_order")
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: <stable@vger.kernel.org>	[5.9+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>