mm, rmap: fix false positive VM_BUG() in page_add_file_rmap()

PageTransCompound() doesn't distinguish THP from from any other type of
compound pages.  This can lead to false-positive VM_BUG_ON() in
page_add_file_rmap() if called on compound page from a driver[1].

I think we can exclude such cases by checking if the page belong to a
mapping.

The VM_BUG_ON_PAGE() is downgraded to VM_WARN_ON_ONCE().  This path
should not cause any harm to non-THP page, but good to know if we step
on anything else.

[1] http://lkml.kernel.org/r/c711e067-0bff-a6cb-3c37-04dfe77d2db1@redhat.com

Link: http://lkml.kernel.org/r/20160810161345.GA67522@black.fi.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: Laura Abbott <labbott@redhat.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -1284,8 +1284,9 @@ void page_add_file_rmap(struct page *page, bool compound)
 		VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
 		__inc_node_page_state(page, NR_SHMEM_PMDMAPPED);
 	} else {
-		if (PageTransCompound(page)) {
-			VM_BUG_ON_PAGE(!PageLocked(page), page);
+		if (PageTransCompound(page) && page_mapping(page)) {
+			VM_WARN_ON_ONCE(!PageLocked(page));
+
 			SetPageDoubleMap(compound_head(page));
 			if (PageMlocked(page))
 				clear_page_mlock(compound_head(page));