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 <npiggin@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 57 insertions, 3 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 1d647ab0ee7..cb94488ab96 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -481,7 +481,7 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
         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)]++;
         }
 
diff --git a/mm/rmap.c b/mm/rmap.c
index 1c1af92732d..850165d32b7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -530,19 +530,51 @@ static void __page_set_anon_rmap(struct page *page,
 }
 
 /**
+ * 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);
 }
 
 /*
@@ -553,10 +585,12 @@ void page_add_anon_rmap(struct page *page,
  *
  * 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);
 }
@@ -573,6 +607,26 @@ void page_add_file_rmap(struct page *page)
                 __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
  * @page: page to remove mapping from