lib: radix-tree: check accounting of existing slot replacement users

The bug in khugepaged fixed earlier in this series shows that radix tree
slot replacement is fragile; and it will become more so when not only
NULL<->!NULL transitions need to be caught but transitions from and to
exceptional entries as well.  We need checks.

Re-implement radix_tree_replace_slot() on top of the sanity-checked
__radix_tree_replace().  This requires existing callers to also pass the
radix tree root, but it'll warn us when somebody replaces slots with
contents that need proper accounting (transitions between NULL entries,
real entries, exceptional entries) and where a replacement through the
slot pointer would corrupt the radix tree node counts.

Link: http://lkml.kernel.org/r/20161117193021.GB23430@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <mawilcox@linuxonhyperv.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/s390/mm/gmap.c

@@ -1015,7 +1015,7 @@ static inline void gmap_insert_rmap(struct gmap *sg, unsigned long vmaddr,
 	if (slot) {
 		rmap->next = radix_tree_deref_slot_protected(slot,
 							&sg->guest_table_lock);
-		radix_tree_replace_slot(slot, rmap);
+		radix_tree_replace_slot(&sg->host_to_rmap, slot, rmap);
 	} else {
 		rmap->next = NULL;
 		radix_tree_insert(&sg->host_to_rmap, vmaddr >> PAGE_SHIFT,

drivers/sh/intc/virq.c

@@ -254,7 +254,7 @@ static void __init intc_subgroup_map(struct intc_desc_int *d)
 
 		radix_tree_tag_clear(&d->tree, entry->enum_id,
 				     INTC_TAG_VIRQ_NEEDS_ALLOC);
-		radix_tree_replace_slot((void **)entries[i],
+		radix_tree_replace_slot(&d->tree, (void **)entries[i],
 					&intc_irq_xlate[irq]);
 	}
 

fs/dax.c

@@ -342,7 +342,7 @@ static inline void *lock_slot(struct address_space *mapping, void **slot)
 		radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
 
 	entry |= RADIX_DAX_ENTRY_LOCK;
-	radix_tree_replace_slot(slot, (void *)entry);
+	radix_tree_replace_slot(&mapping->page_tree, slot, (void *)entry);
 	return (void *)entry;
 }
 
@@ -356,7 +356,7 @@ static inline void *unlock_slot(struct address_space *mapping, void **slot)
 		radix_tree_deref_slot_protected(slot, &mapping->tree_lock);
 
 	entry &= ~(unsigned long)RADIX_DAX_ENTRY_LOCK;
-	radix_tree_replace_slot(slot, (void *)entry);
+	radix_tree_replace_slot(&mapping->page_tree, slot, (void *)entry);
 	return (void *)entry;
 }
 

include/linux/radix-tree.h

@@ -249,20 +249,6 @@ static inline int radix_tree_exception(void *arg)
 	return unlikely((unsigned long)arg & RADIX_TREE_ENTRY_MASK);
 }
 
-/**
- * radix_tree_replace_slot	- replace item in a slot
- * @pslot:	pointer to slot, returned by radix_tree_lookup_slot
- * @item:	new item to store in the slot.
- *
- * For use with radix_tree_lookup_slot().  Caller must hold tree write locked
- * across slot lookup and replacement.
- */
-static inline void radix_tree_replace_slot(void **pslot, void *item)
-{
-	BUG_ON(radix_tree_is_internal_node(item));
-	rcu_assign_pointer(*pslot, item);
-}
-
 int __radix_tree_create(struct radix_tree_root *root, unsigned long index,
 			unsigned order, struct radix_tree_node **nodep,
 			void ***slotp);
@@ -280,6 +266,8 @@ void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long);
 void __radix_tree_replace(struct radix_tree_root *root,
 			  struct radix_tree_node *node,
 			  void **slot, void *item);
+void radix_tree_replace_slot(struct radix_tree_root *root,
+			     void **slot, void *item);
 bool __radix_tree_delete_node(struct radix_tree_root *root,
 			      struct radix_tree_node *node);
 void *radix_tree_delete_item(struct radix_tree_root *, unsigned long, void *);

lib/radix-tree.c

@@ -753,6 +753,28 @@ void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index)
 }
 EXPORT_SYMBOL(radix_tree_lookup);
 
+static void replace_slot(struct radix_tree_root *root,
+			 struct radix_tree_node *node,
+			 void **slot, void *item,
+			 bool warn_typeswitch)
+{
+	void *old = rcu_dereference_raw(*slot);
+	int exceptional;
+
+	WARN_ON_ONCE(radix_tree_is_internal_node(item));
+	WARN_ON_ONCE(!!item - !!old);
+
+	exceptional = !!radix_tree_exceptional_entry(item) -
+		      !!radix_tree_exceptional_entry(old);
+
+	WARN_ON_ONCE(warn_typeswitch && exceptional);
+
+	if (node)
+		node->exceptional += exceptional;
+
+	rcu_assign_pointer(*slot, item);
+}
+
 /**
  * __radix_tree_replace		- replace item in a slot
  * @root:	radix tree root
@@ -767,21 +789,34 @@ void __radix_tree_replace(struct radix_tree_root *root,
 			  struct radix_tree_node *node,
 			  void **slot, void *item)
 {
-	void *old = rcu_dereference_raw(*slot);
-	int exceptional;
+	/*
+	 * This function supports replacing exceptional entries, but
+	 * that needs accounting against the node unless the slot is
+	 * root->rnode.
+	 */
+	replace_slot(root, node, slot, item,
+		     !node && slot != (void **)&root->rnode);
+}
 
-	WARN_ON_ONCE(radix_tree_is_internal_node(item));
-	WARN_ON_ONCE(!!item - !!old);
-
-	exceptional = !!radix_tree_exceptional_entry(item) -
-		      !!radix_tree_exceptional_entry(old);
-
-	WARN_ON_ONCE(exceptional && !node && slot != (void **)&root->rnode);
-
-	if (node)
-		node->exceptional += exceptional;
-
-	rcu_assign_pointer(*slot, item);
+/**
+ * radix_tree_replace_slot	- replace item in a slot
+ * @root:	radix tree root
+ * @slot:	pointer to slot
+ * @item:	new item to store in the slot.
+ *
+ * For use with radix_tree_lookup_slot(), radix_tree_gang_lookup_slot(),
+ * radix_tree_gang_lookup_tag_slot().  Caller must hold tree write locked
+ * across slot lookup and replacement.
+ *
+ * NOTE: This cannot be used to switch between non-entries (empty slots),
+ * regular entries, and exceptional entries, as that requires accounting
+ * inside the radix tree node. When switching from one type of entry to
+ * another, use __radix_tree_lookup() and __radix_tree_replace().
+ */
+void radix_tree_replace_slot(struct radix_tree_root *root,
+			     void **slot, void *item)
+{
+	replace_slot(root, NULL, slot, item, true);
 }
 
 /**

mm/filemap.c

@@ -147,7 +147,7 @@ static int page_cache_tree_insert(struct address_space *mapping,
 						      false);
 		}
 	}
-	radix_tree_replace_slot(slot, page);
+	radix_tree_replace_slot(&mapping->page_tree, slot, page);
 	mapping->nrpages++;
 	if (node) {
 		workingset_node_pages_inc(node);
@@ -196,7 +196,7 @@ static void page_cache_tree_delete(struct address_space *mapping,
 			shadow = NULL;
 		}
 
-		radix_tree_replace_slot(slot, shadow);
+		radix_tree_replace_slot(&mapping->page_tree, slot, shadow);
 
 		if (!node)
 			break;

mm/khugepaged.c

@@ -1426,7 +1426,7 @@ static void collapse_shmem(struct mm_struct *mm,
 		list_add_tail(&page->lru, &pagelist);
 
 		/* Finally, replace with the new page. */
-		radix_tree_replace_slot(slot,
+		radix_tree_replace_slot(&mapping->page_tree, slot,
 				new_page + (index % HPAGE_PMD_NR));
 
 		slot = radix_tree_iter_next(&iter);
@@ -1538,7 +1538,8 @@ static void collapse_shmem(struct mm_struct *mm,
 			/* Unfreeze the page. */
 			list_del(&page->lru);
 			page_ref_unfreeze(page, 2);
-			radix_tree_replace_slot(slot, page);
+			radix_tree_replace_slot(&mapping->page_tree,
+						slot, page);
 			spin_unlock_irq(&mapping->tree_lock);
 			putback_lru_page(page);
 			unlock_page(page);

mm/migrate.c

@@ -482,7 +482,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
 		SetPageDirty(newpage);
 	}
 
-	radix_tree_replace_slot(pslot, newpage);
+	radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);
 
 	/*
 	 * Drop cache reference from old page by unfreezing
@@ -556,7 +556,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
 
 	get_page(newpage);
 
-	radix_tree_replace_slot(pslot, newpage);
+	radix_tree_replace_slot(&mapping->page_tree, pslot, newpage);
 
 	page_ref_unfreeze(page, expected_count - 1);
 

mm/truncate.c

@@ -49,7 +49,7 @@ static void clear_exceptional_entry(struct address_space *mapping,
 		goto unlock;
 	if (*slot != entry)
 		goto unlock;
-	radix_tree_replace_slot(slot, NULL);
+	radix_tree_replace_slot(&mapping->page_tree, slot, NULL);
 	mapping->nrexceptional--;
 	if (!node)
 		goto unlock;

tools/testing/radix-tree/multiorder.c

@@ -146,7 +146,7 @@ static void multiorder_check(unsigned long index, int order)
 
 	slot = radix_tree_lookup_slot(&tree, index);
 	free(*slot);
-	radix_tree_replace_slot(slot, item2);
+	radix_tree_replace_slot(&tree, slot, item2);
 	for (i = min; i < max; i++) {
 		struct item *item = item_lookup(&tree, i);
 		assert(item != 0);