mm: drop bogus VM_BUG_ON_PAGE assert in put_page() codepath

My commit 8d63d99a5d ("mm: avoid tail page refcounting on non-THP
compound pages") which was merged during 4.1 merge window caused
regression:

  page:ffffea0010a15040 count:0 mapcount:1 mapping:          (null) index:0x0
  flags: 0x8000000000008014(referenced|dirty|tail)
  page dumped because: VM_BUG_ON_PAGE(page_mapcount(page) != 0)
  ------------[ cut here ]------------
  kernel BUG at mm/swap.c:134!

The problem can be reproduced by playing *two* audio files at the same
time and then stopping one of players.  I used two mplayers to trigger
this.

The VM_BUG_ON_PAGE() which triggers the bug is bogus:

Sound subsystem uses compound pages for its buffers, but unlike most
__GFP_COMP sound maps compound pages to userspace with PTEs.

In our case with two players map the buffer twice and therefore elevates
page_mapcount() on tail pages by two.  When one of players exits it
unmaps the VMA and drops page_mapcount() to one and try to release
reference on the page with put_page().

My commit changes which path it takes under put_compound_page().  It hits
put_unrefcounted_compound_page() where VM_BUG_ON_PAGE() is.  It sees
page_mapcount() == 1.  The function wrongly assumes that subpages of
compound page cannot be be mapped by itself with PTEs..

The solution is simply drop the VM_BUG_ON_PAGE().

Note: there's no need to move the check under put_page_testzero().
Allocator will check the mapcount by itself before putting on free list.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap.c

@@ -131,7 +131,6 @@ void put_unrefcounted_compound_page(struct page *page_head, struct page *page)
 		 * here, see the comment above this function.
 		 */
 		VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
-		VM_BUG_ON_PAGE(page_mapcount(page) != 0, page);
 		if (put_page_testzero(page_head)) {
 			/*
 			 * If this is the tail of a slab THP page,