lib/stackdepot: annotate init and early init functions

Add comments to stack_depot_early_init and stack_depot_init to explain
certain parts of their implementation.

Also add a pr_info message to stack_depot_early_init similar to the one
in stack_depot_init.

Also move the scale variable in stack_depot_init to the scope where it
is being used.

Link: https://lkml.kernel.org/r/d17fbfbd4d73f38686c5e3d4824a6d62047213a1.1676063693.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/stackdepot.c

@@ -115,24 +115,34 @@ void __init stack_depot_request_early_init(void)
 	__stack_depot_early_init_requested = true;
 }
 
+/* Allocates a hash table via memblock. Can only be used during early boot. */
 int __init stack_depot_early_init(void)
 {
 	unsigned long entries = 0;
 
-	/* This is supposed to be called only once, from mm_init() */
+	/* This function must be called only once, from mm_init(). */
 	if (WARN_ON(__stack_depot_early_init_passed))
 		return 0;
-
 	__stack_depot_early_init_passed = true;
 
+	/*
+	 * If KASAN is enabled, use the maximum order: KASAN is frequently used
+	 * in fuzzing scenarios, which leads to a large number of different
+	 * stack traces being stored in stack depot.
+	 */
 	if (kasan_enabled() && !stack_hash_order)
 		stack_hash_order = STACK_HASH_ORDER_MAX;
 
 	if (!__stack_depot_early_init_requested || stack_depot_disabled)
 		return 0;
 
+	/*
+	 * If stack_hash_order is not set, leave entries as 0 to rely on the
+	 * automatic calculations performed by alloc_large_system_hash.
+	 */
 	if (stack_hash_order)
-		entries = 1UL <<  stack_hash_order;
+		entries = 1UL << stack_hash_order;
+	pr_info("allocating hash table via alloc_large_system_hash\n");
 	stack_table = alloc_large_system_hash("stackdepot",
 						sizeof(struct stack_record *),
 						entries,
@@ -142,7 +152,6 @@ int __init stack_depot_early_init(void)
 						&stack_hash_mask,
 						1UL << STACK_HASH_ORDER_MIN,
 						1UL << STACK_HASH_ORDER_MAX);
-
 	if (!stack_table) {
 		pr_err("hash table allocation failed, disabling\n");
 		stack_depot_disabled = true;
@@ -152,6 +161,7 @@ int __init stack_depot_early_init(void)
 	return 0;
 }
 
+/* Allocates a hash table via kvcalloc. Can be used after boot. */
 int stack_depot_init(void)
 {
 	static DEFINE_MUTEX(stack_depot_init_mutex);
@@ -160,11 +170,16 @@ int stack_depot_init(void)
 	mutex_lock(&stack_depot_init_mutex);
 	if (!stack_depot_disabled && !stack_table) {
 		unsigned long entries;
-		int scale = STACK_HASH_SCALE;
 
+		/*
+		 * Similarly to stack_depot_early_init, use stack_hash_order
+		 * if assigned, and rely on automatic scaling otherwise.
+		 */
 		if (stack_hash_order) {
 			entries = 1UL << stack_hash_order;
 		} else {
+			int scale = STACK_HASH_SCALE;
+
 			entries = nr_free_buffer_pages();
 			entries = roundup_pow_of_two(entries);
 
@@ -179,7 +194,7 @@ int stack_depot_init(void)
 		if (entries > 1UL << STACK_HASH_ORDER_MAX)
 			entries = 1UL << STACK_HASH_ORDER_MAX;
 
-		pr_info("allocating hash table of %lu entries with kvcalloc\n",
+		pr_info("allocating hash table of %lu entries via kvcalloc\n",
 				entries);
 		stack_table = kvcalloc(entries, sizeof(struct stack_record *), GFP_KERNEL);
 		if (!stack_table) {