mm: more rmap checking

Re-introduce rmap verification patches that Hugh removed when he removed
PG_map_lock. PG_map_lock actually isn't needed to synchronise access to
anonymous pages, because PG_locked and PTL together already do.

These checks were important in discovering and fixing a rare rmap corruption
in SLES9.

Signed-off-by: Nick Piggin <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/rmap.h

···
 void page_add_file_rmap(struct page *);
 void page_remove_rmap(struct page *, struct vm_area_struct *);
 
-/**
- * page_dup_rmap - duplicate pte mapping to a page
- * @page:	the page to add the mapping to
- *
- * For copy_page_range only: minimal extract from page_add_rmap,
- * avoiding unnecessary tests (already checked) so it's quicker.
- */
-static inline void page_dup_rmap(struct page *page)
+#ifdef CONFIG_DEBUG_VM
+void page_dup_rmap(struct page *page, struct vm_area_struct *vma, unsigned long address);
+#else
+static inline void page_dup_rmap(struct page *page, struct vm_area_struct *vma, unsigned long address)
 {
 	atomic_inc(&page->_mapcount);
 }
+#endif
 
 /*
  * Called from mm/vmscan.c to handle paging out

mm/memory.c

···
 	page = vm_normal_page(vma, addr, pte);
 	if (page) {
 		get_page(page);
-		page_dup_rmap(page);
+		page_dup_rmap(page, vma, addr);
 		rss[!!PageAnon(page)]++;
 	}
 

mm/rmap.c

···
 }
 
 /**
+ * page_set_anon_rmap - sanity check anonymous rmap addition
+ * @page:	the page to add the mapping to
+ * @vma:	the vm area in which the mapping is added
+ * @address:	the user virtual address mapped
+ */
+static void __page_check_anon_rmap(struct page *page,
+	struct vm_area_struct *vma, unsigned long address)
+{
+#ifdef CONFIG_DEBUG_VM
+	/*
+	 * The page's anon-rmap details (mapping and index) are guaranteed to
+	 * be set up correctly at this point.
+	 *
+	 * We have exclusion against page_add_anon_rmap because the caller
+	 * always holds the page locked, except if called from page_dup_rmap,
+	 * in which case the page is already known to be setup.
+	 *
+	 * We have exclusion against page_add_new_anon_rmap because those pages
+	 * are initially only visible via the pagetables, and the pte is locked
+	 * over the call to page_add_new_anon_rmap.
+	 */
+	struct anon_vma *anon_vma = vma->anon_vma;
+	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
+	BUG_ON(page->mapping != (struct address_space *)anon_vma);
+	BUG_ON(page->index != linear_page_index(vma, address));
+#endif
+}
+
+/**
  * page_add_anon_rmap - add pte mapping to an anonymous page
  * @page:	the page to add the mapping to
  * @vma:	the vm area in which the mapping is added
  * @address:	the user virtual address mapped
  *
- * The caller needs to hold the pte lock.
+ * The caller needs to hold the pte lock and the page must be locked.
  */
 void page_add_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address)
 {
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	if (atomic_inc_and_test(&page->_mapcount))
 		__page_set_anon_rmap(page, vma, address);
-	/* else checking page index and mapping is racy */
+	else
+		__page_check_anon_rmap(page, vma, address);
 }
 
 /*
···
  *
  * Same as page_add_anon_rmap but must only be called on *new* pages.
  * This means the inc-and-test can be bypassed.
+ * Page does not have to be locked.
  */
 void page_add_new_anon_rmap(struct page *page,
 	struct vm_area_struct *vma, unsigned long address)
 {
+	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
 	atomic_set(&page->_mapcount, 0); /* elevate count by 1 (starts at -1) */
 	__page_set_anon_rmap(page, vma, address);
 }
···
 	if (atomic_inc_and_test(&page->_mapcount))
 		__inc_zone_page_state(page, NR_FILE_MAPPED);
 }
+
+#ifdef CONFIG_DEBUG_VM
+/**
+ * page_dup_rmap - duplicate pte mapping to a page
+ * @page:	the page to add the mapping to
+ *
+ * For copy_page_range only: minimal extract from page_add_file_rmap /
+ * page_add_anon_rmap, avoiding unnecessary tests (already checked) so it's
+ * quicker.
+ *
+ * The caller needs to hold the pte lock.
+ */
+void page_dup_rmap(struct page *page, struct vm_area_struct *vma, unsigned long address)
+{
+	BUG_ON(page_mapcount(page) == 0);
+	if (PageAnon(page))
+		__page_check_anon_rmap(page, vma, address);
+	atomic_inc(&page->_mapcount);
+}
+#endif
 
 /**
  * page_remove_rmap - take down pte mapping from a page