mm: thp: refix false positive BUG in page_move_anon_rmap()

The VM_BUG_ON_PAGE in page_move_anon_rmap() is more trouble than it's
worth: the syzkaller fuzzer hit it again.  It's still wrong for some THP
cases, because linear_page_index() was never intended to apply to
addresses before the start of a vma.

That's easily fixed with a signed long cast inside linear_page_index();
and Dmitry has tested such a patch, to verify the false positive.  But
why extend linear_page_index() just for this case? when the avoidance in
page_move_anon_rmap() has already grown ugly, and there's no reason for
the check at all (nothing else there is using address or index).

Remove address arg from page_move_anon_rmap(), remove VM_BUG_ON_PAGE,
remove CONFIG_DEBUG_VM PageTransHuge adjustment.

And one more thing: should the compound_head(page) be done inside or
outside page_move_anon_rmap()? It's usually pushed down to the lowest
level nowadays (and mm/memory.c shows no other explicit use of it), so I
think it's better done in page_move_anon_rmap() than by caller.

Fixes: 0798d3c022dc ("mm: thp: avoid false positive VM_BUG_ON_PAGE in page_move_anon_rmap()")
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1607120444540.12528@eggly.anvils
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: <stable@vger.kernel.org>	[4.5+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 6 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index e4b713a6ed7e..701b93fea2a0 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1084,23 +1084,20 @@ EXPORT_SYMBOL_GPL(page_mkclean);
  * page_move_anon_rmap - move a page to our anon_vma
  * @page:       the page to move to our anon_vma
  * @vma:        the vma the page belongs to
- * @address:    the user virtual address mapped
  *
  * When a page belongs exclusively to one process after a COW event,
  * that page can be moved into the anon_vma that belongs to just that
  * process, so the rmap code will not search the parent or sibling
  * processes.
  */
-void page_move_anon_rmap(struct page *page,
-        struct vm_area_struct *vma, unsigned long address)
+void page_move_anon_rmap(struct page *page, struct vm_area_struct *vma)
 {
         struct anon_vma *anon_vma = vma->anon_vma;
 
+        page = compound_head(page);
+
         VM_BUG_ON_PAGE(!PageLocked(page), page);
         VM_BUG_ON_VMA(!anon_vma, vma);
-        if (IS_ENABLED(CONFIG_DEBUG_VM) && PageTransHuge(page))
-                address &= HPAGE_PMD_MASK;
-        VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page);
 
         anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
         /*