maple_tree: add test to replicate low memory race conditions

Add new callback fields to the userspace implementation of struct
kmem_cache.  This allows for executing callback functions in order to
further test low memory scenarios where node allocation is retried.

This callback can help test race conditions by calling a function when a
low memory event is tested.

Link: https://lkml.kernel.org/r/20240812190543.71967-2-sidhartha.kumar@oracle.comSigned-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -7005,6 +7005,19 @@ void mt_set_non_kernel(unsigned int val)
 	kmem_cache_set_non_kernel(maple_node_cache, val);
 }
 
+extern void kmem_cache_set_callback(struct kmem_cache *cachep,
+		void (*callback)(void *));
+void mt_set_callback(void (*callback)(void *))
+{
+	kmem_cache_set_callback(maple_node_cache, callback);
+}
+
+extern void kmem_cache_set_private(struct kmem_cache *cachep, void *private);
+void mt_set_private(void *private)
+{
+	kmem_cache_set_private(maple_node_cache, private);
+}
+
 extern unsigned long kmem_cache_get_alloc(struct kmem_cache *);
 unsigned long mt_get_alloc_size(void)
 {

tools/testing/radix-tree/maple.c

@@ -36224,6 +36224,65 @@ static noinline void __init check_mtree_dup(struct maple_tree *mt)
 
 extern void test_kmem_cache_bulk(void);
 
+/* callback function used for check_nomem_writer_race() */
+static void writer2(void *maple_tree)
+{
+	struct maple_tree *mt = (struct maple_tree *)maple_tree;
+	MA_STATE(mas, mt, 6, 10);
+
+	mtree_lock(mas.tree);
+	mas_store(&mas, xa_mk_value(0xC));
+	mas_destroy(&mas);
+	mtree_unlock(mas.tree);
+}
+
+/*
+ * check_nomem_writer_race() - test a possible race in the mas_nomem() path
+ * @mt: The tree to build.
+ *
+ * There is a possible race condition in low memory conditions when mas_nomem()
+ * gives up its lock. A second writer can chagne the entry that the primary
+ * writer executing the mas_nomem() path is modifying. This test recreates this
+ * scenario to ensure we are handling it correctly.
+ */
+static void check_nomem_writer_race(struct maple_tree *mt)
+{
+	MA_STATE(mas, mt, 0, 5);
+
+	mt_set_non_kernel(0);
+	/* setup root with 2 values with NULL in between */
+	mtree_store_range(mt, 0, 5, xa_mk_value(0xA), GFP_KERNEL);
+	mtree_store_range(mt, 6, 10, NULL, GFP_KERNEL);
+	mtree_store_range(mt, 11, 15, xa_mk_value(0xB), GFP_KERNEL);
+
+	/* setup writer 2 that will trigger the race condition */
+	mt_set_private(mt);
+	mt_set_callback(writer2);
+
+	mtree_lock(mt);
+	/* erase 0-5 */
+	mas_erase(&mas);
+
+	/* index 6-10 should retain the value from writer 2 */
+	check_load(mt, 6, xa_mk_value(0xC));
+	mtree_unlock(mt);
+
+	/* test for the same race but with mas_store_gfp() */
+	mtree_store_range(mt, 0, 5, xa_mk_value(0xA), GFP_KERNEL);
+	mtree_store_range(mt, 6, 10, NULL, GFP_KERNEL);
+
+	mas_set_range(&mas, 0, 5);
+	mtree_lock(mt);
+	mas_store_gfp(&mas, NULL, GFP_KERNEL);
+
+	/* ensure write made by writer 2 is retained */
+	check_load(mt, 6, xa_mk_value(0xC));
+
+	mt_set_private(NULL);
+	mt_set_callback(NULL);
+	mtree_unlock(mt);
+}
+
 void farmer_tests(void)
 {
 	struct maple_node *node;
@@ -36257,6 +36316,10 @@ void farmer_tests(void)
 	check_dfs_preorder(&tree);
 	mtree_destroy(&tree);
 
+	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE | MT_FLAGS_USE_RCU);
+	check_nomem_writer_race(&tree);
+	mtree_destroy(&tree);
+
 	mt_init_flags(&tree, MT_FLAGS_ALLOC_RANGE);
 	check_prealloc(&tree);
 	mtree_destroy(&tree);

tools/testing/shared/linux.c

@@ -26,8 +26,21 @@ struct kmem_cache {
 	unsigned int non_kernel;
 	unsigned long nr_allocated;
 	unsigned long nr_tallocated;
+	bool exec_callback;
+	void (*callback)(void *);
+	void *private;
 };
 
+void kmem_cache_set_callback(struct kmem_cache *cachep, void (*callback)(void *))
+{
+	cachep->callback = callback;
+}
+
+void kmem_cache_set_private(struct kmem_cache *cachep, void *private)
+{
+	cachep->private = private;
+}
+
 void kmem_cache_set_non_kernel(struct kmem_cache *cachep, unsigned int val)
 {
 	cachep->non_kernel = val;
@@ -58,9 +71,17 @@ void *kmem_cache_alloc_lru(struct kmem_cache *cachep, struct list_lru *lru,
 {
 	void *p;
 
+	if (cachep->exec_callback) {
+		if (cachep->callback)
+			cachep->callback(cachep->private);
+		cachep->exec_callback = false;
+	}
+
 	if (!(gfp & __GFP_DIRECT_RECLAIM)) {
-		if (!cachep->non_kernel)
+		if (!cachep->non_kernel) {
+			cachep->exec_callback = true;
 			return NULL;
+		}
 
 		cachep->non_kernel--;
 	}
@@ -223,6 +244,9 @@ kmem_cache_create(const char *name, unsigned int size, unsigned int align,
 	ret->objs = NULL;
 	ret->ctor = ctor;
 	ret->non_kernel = 0;
+	ret->exec_callback = false;
+	ret->callback = NULL;
+	ret->private = NULL;
 	return ret;
 }