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>

1 files changed, 3 insertions, 2 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 709bc83703b1..d4f56060ba3f 100644
--- a/mm/rmap.c
+++ b/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));