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: 0798d3c022 ("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>

include/linux/rmap.h

@@ -158,7 +158,7 @@ struct anon_vma *page_get_anon_vma(struct page *page);
 /*
  * rmap interfaces called when adding or removing pte of page
  */
-void page_move_anon_rmap(struct page *, struct vm_area_struct *, unsigned long);
+void page_move_anon_rmap(struct page *, struct vm_area_struct *);
 void page_add_anon_rmap(struct page *, struct vm_area_struct *,
 		unsigned long, bool);
 void do_page_add_anon_rmap(struct page *, struct vm_area_struct *,

mm/hugetlb.c

@@ -3383,7 +3383,7 @@ static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
 	/* If no-one else is actually using this page, avoid the copy
 	 * and just make the page writable */
 	if (page_mapcount(old_page) == 1 && PageAnon(old_page)) {
-		page_move_anon_rmap(old_page, vma, address);
+		page_move_anon_rmap(old_page, vma);
 		set_huge_ptep_writable(vma, address, ptep);
 		return 0;
 	}

mm/memory.c

@@ -2399,8 +2399,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				 * Protected against the rmap code by
 				 * the page lock.
 				 */
-				page_move_anon_rmap(compound_head(old_page),
-						    vma, address);
+				page_move_anon_rmap(old_page, vma);
 			}
 			unlock_page(old_page);
 			return wp_page_reuse(mm, vma, address, page_table, ptl,

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;
 	/*