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Date:   Tue, 28 Nov 2023 16:04:40 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/kmemleak: Add cond_resched() to kmemleak_free_percpu()

On Mon, Nov 27, 2023 at 02:41:53PM -0500, Waiman Long wrote:
>  /**
>   * kmemleak_free_percpu - unregister a previously registered __percpu object
>   * @ptr:	__percpu pointer to beginning of the object
>   *
>   * This function is called from the kernel percpu allocator when an object
> - * (memory block) is freed (free_percpu).
> + * (memory block) is freed (free_percpu). Since this function is inherently
> + * slow especially on systems with a large number of CPUs, defer the actual
> + * removal of kmemleak objects associated with the percpu pointer to a
> + * workqueue if it is not in a task context.
>   */
>  void __ref kmemleak_free_percpu(const void __percpu *ptr)
>  {
> -	unsigned int cpu;
> -
>  	pr_debug("%s(0x%px)\n", __func__, ptr);
>  
> -	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
> -		for_each_possible_cpu(cpu)
> -			delete_object_full((unsigned long)per_cpu_ptr(ptr,
> -								      cpu));
> +	if (!kmemleak_free_enabled || !ptr || IS_ERR(ptr))
> +		return;
> +
> +	if (!in_task()) {
> +		struct kmemleak_percpu_addr *addr;
> +
> +		addr = kzalloc(sizeof(*addr), GFP_ATOMIC);
> +		if (addr) {
> +			INIT_WORK(&addr->work, kmemleak_free_percpu_workfn);
> +			addr->ptr = ptr;
> +			queue_work(system_long_wq, &addr->work);
> +			return;
> +		}

We can't defer this freeing. It can mess up the kmemleak metadata if the
per-cpu pointer is re-allocated before kmemleak removed it from its
object tree.

The problem is looking up the object tree for each per-cpu offset. We
can make the percpu pointer handling O(1) since freeing is only done by
the main __percpu pointer, so that's the only one needing a look-up. So
far the per-cpu pointers are not tracked for leaking, only scanned.

We could just add the per_cpu_ptr(ptr, 0) to the kmemleak
object_tree_root but when scanning we don't have an inverse function to
get the __percpu pointer back and calculate the pointers for the other
CPUs (well, we could with some hacks but they are probably fragile).

What I came up with is a separate object_percpu_tree_root similar to the
object_phys_tree_root. The only reason for these additional trees is to
look up the kmemleak metadata when needed (usually freeing). They don't
contain objects that are tracked for actual leaking, only scanned. A
briefly tested patch below. I need to go through it again, update some
comments and write a commit log:

---------------------8<---------------------------------
diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 1eacca03bedd..7446c9e0b8c8 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -178,6 +178,8 @@ struct kmemleak_object {
 #define OBJECT_FULL_SCAN	(1 << 3)
 /* flag set for object allocated with physical address */
 #define OBJECT_PHYS		(1 << 4)
+/* flag set for per-CPU pointers */
+#define OBJECT_PERCPU		(1 << 5)
 
 /* set when __remove_object() called */
 #define DELSTATE_REMOVED	(1 << 0)
@@ -206,6 +208,8 @@ static LIST_HEAD(mem_pool_free_list);
 static struct rb_root object_tree_root = RB_ROOT;
 /* search tree for object (with OBJECT_PHYS flag) boundaries */
 static struct rb_root object_phys_tree_root = RB_ROOT;
+/* search tree for object (with OBJECT_PERCPU flag) boundaries */
+static struct rb_root object_percpu_tree_root = RB_ROOT;
 /* protecting the access to object_list, object_tree_root (or object_phys_tree_root) */
 static DEFINE_RAW_SPINLOCK(kmemleak_lock);
 
@@ -298,7 +302,7 @@ static void hex_dump_object(struct seq_file *seq,
 	const u8 *ptr = (const u8 *)object->pointer;
 	size_t len;
 
-	if (WARN_ON_ONCE(object->flags & OBJECT_PHYS))
+	if (WARN_ON_ONCE(object->flags & (OBJECT_PHYS | OBJECT_PERCPU)))
 		return;
 
 	/* limit the number of lines to HEX_MAX_LINES */
@@ -392,6 +396,15 @@ static void dump_object_info(struct kmemleak_object *object)
 		stack_depot_print(object->trace_handle);
 }
 
+static struct rb_root *object_tree(unsigned long objflags)
+{
+	if (objflags & OBJECT_PHYS)
+		return &object_phys_tree_root;
+	if (objflags & OBJECT_PERCPU)
+		return &object_percpu_tree_root;
+	return &object_tree_root;
+}
+
 /*
  * Look-up a memory block metadata (kmemleak_object) in the object search
  * tree based on a pointer value. If alias is 0, only values pointing to the
@@ -399,10 +412,9 @@ static void dump_object_info(struct kmemleak_object *object)
  * when calling this function.
  */
 static struct kmemleak_object *__lookup_object(unsigned long ptr, int alias,
-					       bool is_phys)
+					       unsigned int objflags)
 {
-	struct rb_node *rb = is_phys ? object_phys_tree_root.rb_node :
-			     object_tree_root.rb_node;
+	struct rb_node *rb = object_tree(objflags)->rb_node;
 	unsigned long untagged_ptr = (unsigned long)kasan_reset_tag((void *)ptr);
 
 	while (rb) {
@@ -431,7 +443,7 @@ static struct kmemleak_object *__lookup_object(unsigned long ptr, int alias,
 /* Look-up a kmemleak object which allocated with virtual address. */
 static struct kmemleak_object *lookup_object(unsigned long ptr, int alias)
 {
-	return __lookup_object(ptr, alias, false);
+	return __lookup_object(ptr, alias, 0);
 }
 
 /*
@@ -544,14 +556,14 @@ static void put_object(struct kmemleak_object *object)
  * Look up an object in the object search tree and increase its use_count.
  */
 static struct kmemleak_object *__find_and_get_object(unsigned long ptr, int alias,
-						     bool is_phys)
+						     unsigned int objflags)
 {
 	unsigned long flags;
 	struct kmemleak_object *object;
 
 	rcu_read_lock();
 	raw_spin_lock_irqsave(&kmemleak_lock, flags);
-	object = __lookup_object(ptr, alias, is_phys);
+	object = __lookup_object(ptr, alias, objflags);
 	raw_spin_unlock_irqrestore(&kmemleak_lock, flags);
 
 	/* check whether the object is still available */
@@ -565,7 +577,7 @@ static struct kmemleak_object *__find_and_get_object(unsigned long ptr, int alia
 /* Look up and get an object which allocated with virtual address. */
 static struct kmemleak_object *find_and_get_object(unsigned long ptr, int alias)
 {
-	return __find_and_get_object(ptr, alias, false);
+	return __find_and_get_object(ptr, alias, 0);
 }
 
 /*
@@ -575,9 +587,7 @@ static struct kmemleak_object *find_and_get_object(unsigned long ptr, int alias)
  */
 static void __remove_object(struct kmemleak_object *object)
 {
-	rb_erase(&object->rb_node, object->flags & OBJECT_PHYS ?
-				   &object_phys_tree_root :
-				   &object_tree_root);
+	rb_erase(&object->rb_node, object_tree(object->flags));
 	if (!(object->del_state & DELSTATE_NO_DELETE))
 		list_del_rcu(&object->object_list);
 	object->del_state |= DELSTATE_REMOVED;
@@ -585,11 +595,11 @@ static void __remove_object(struct kmemleak_object *object)
 
 static struct kmemleak_object *__find_and_remove_object(unsigned long ptr,
 							int alias,
-							bool is_phys)
+							unsigned int objflags)
 {
 	struct kmemleak_object *object;
 
-	object = __lookup_object(ptr, alias, is_phys);
+	object = __lookup_object(ptr, alias, objflags);
 	if (object)
 		__remove_object(object);
 
@@ -603,13 +613,13 @@ static struct kmemleak_object *__find_and_remove_object(unsigned long ptr,
  * by create_object().
  */
 static struct kmemleak_object *find_and_remove_object(unsigned long ptr, int alias,
-						      bool is_phys)
+						      unsigned int objflags)
 {
 	unsigned long flags;
 	struct kmemleak_object *object;
 
 	raw_spin_lock_irqsave(&kmemleak_lock, flags);
-	object = __find_and_remove_object(ptr, alias, is_phys);
+	object = __find_and_remove_object(ptr, alias, objflags);
 	raw_spin_unlock_irqrestore(&kmemleak_lock, flags);
 
 	return object;
@@ -648,7 +658,7 @@ static struct kmemleak_object *__alloc_object(gfp_t gfp)
 }
 
 static int __link_object(struct kmemleak_object *object, unsigned long ptr,
-			 size_t size, int min_count, bool is_phys)
+			 size_t size, int min_count, unsigned int objflags)
 {
 
 	struct kmemleak_object *parent;
@@ -661,7 +671,7 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr,
 	INIT_HLIST_HEAD(&object->area_list);
 	raw_spin_lock_init(&object->lock);
 	atomic_set(&object->use_count, 1);
-	object->flags = OBJECT_ALLOCATED | (is_phys ? OBJECT_PHYS : 0);
+	object->flags = OBJECT_ALLOCATED | objflags;
 	object->pointer = ptr;
 	object->size = kfence_ksize((void *)ptr) ?: size;
 	object->excess_ref = 0;
@@ -697,12 +707,11 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr,
 	 * Only update min_addr and max_addr with object
 	 * storing virtual address.
 	 */
-	if (!is_phys) {
+	if (!(objflags & (OBJECT_PHYS | OBJECT_PERCPU))) {
 		min_addr = min(min_addr, untagged_ptr);
 		max_addr = max(max_addr, untagged_ptr + size);
 	}
-	link = is_phys ? &object_phys_tree_root.rb_node :
-		&object_tree_root.rb_node;
+	link = &object_tree(objflags)->rb_node;
 	rb_parent = NULL;
 	while (*link) {
 		rb_parent = *link;
@@ -724,8 +733,7 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr,
 		}
 	}
 	rb_link_node(&object->rb_node, rb_parent, link);
-	rb_insert_color(&object->rb_node, is_phys ? &object_phys_tree_root :
-					  &object_tree_root);
+	rb_insert_color(&object->rb_node, object_tree(objflags));
 	list_add_tail_rcu(&object->object_list, &object_list);
 
 	return 0;
@@ -737,7 +745,7 @@ static int __link_object(struct kmemleak_object *object, unsigned long ptr,
  * object_phys_tree_root).
  */
 static void __create_object(unsigned long ptr, size_t size,
-				int min_count, gfp_t gfp, bool is_phys)
+				int min_count, gfp_t gfp, unsigned int objflags)
 {
 	struct kmemleak_object *object;
 	unsigned long flags;
@@ -748,7 +756,7 @@ static void __create_object(unsigned long ptr, size_t size,
 		return;
 
 	raw_spin_lock_irqsave(&kmemleak_lock, flags);
-	ret = __link_object(object, ptr, size, min_count, is_phys);
+	ret = __link_object(object, ptr, size, min_count, objflags);
 	raw_spin_unlock_irqrestore(&kmemleak_lock, flags);
 	if (ret)
 		mem_pool_free(object);
@@ -758,14 +766,21 @@ static void __create_object(unsigned long ptr, size_t size,
 static void create_object(unsigned long ptr, size_t size,
 			  int min_count, gfp_t gfp)
 {
-	__create_object(ptr, size, min_count, gfp, false);
+	__create_object(ptr, size, min_count, gfp, 0);
 }
 
 /* Create kmemleak object which allocated with physical address. */
 static void create_object_phys(unsigned long ptr, size_t size,
 			       int min_count, gfp_t gfp)
 {
-	__create_object(ptr, size, min_count, gfp, true);
+	__create_object(ptr, size, min_count, gfp, OBJECT_PHYS);
+}
+
+/* Create kmemleak object corresponding to a per-CPU allocation. */
+static void create_object_percpu(unsigned long ptr, size_t size,
+				 int min_count, gfp_t gfp)
+{
+	__create_object(ptr, size, min_count, gfp, OBJECT_PERCPU);
 }
 
 /*
@@ -792,11 +807,11 @@ static void __delete_object(struct kmemleak_object *object)
  * Look up the metadata (struct kmemleak_object) corresponding to ptr and
  * delete it.
  */
-static void delete_object_full(unsigned long ptr)
+static void delete_object_full(unsigned long ptr, unsigned int objflags)
 {
 	struct kmemleak_object *object;
 
-	object = find_and_remove_object(ptr, 0, false);
+	object = find_and_remove_object(ptr, 0, objflags);
 	if (!object) {
 #ifdef DEBUG
 		kmemleak_warn("Freeing unknown object at 0x%08lx\n",
@@ -812,7 +827,8 @@ static void delete_object_full(unsigned long ptr)
  * delete it. If the memory block is partially freed, the function may create
  * additional metadata for the remaining parts of the block.
  */
-static void delete_object_part(unsigned long ptr, size_t size, bool is_phys)
+static void delete_object_part(unsigned long ptr, size_t size,
+			       unsigned int objflags)
 {
 	struct kmemleak_object *object, *object_l, *object_r;
 	unsigned long start, end, flags;
@@ -826,7 +842,7 @@ static void delete_object_part(unsigned long ptr, size_t size, bool is_phys)
 		goto out;
 
 	raw_spin_lock_irqsave(&kmemleak_lock, flags);
-	object = __find_and_remove_object(ptr, 1, is_phys);
+	object = __find_and_remove_object(ptr, 1, objflags);
 	if (!object) {
 #ifdef DEBUG
 		kmemleak_warn("Partially freeing unknown object at 0x%08lx (size %zu)\n",
@@ -844,11 +860,11 @@ static void delete_object_part(unsigned long ptr, size_t size, bool is_phys)
 	end = object->pointer + object->size;
 	if ((ptr > start) &&
 	    !__link_object(object_l, start, ptr - start,
-			   object->min_count, is_phys))
+			   object->min_count, objflags))
 		object_l = NULL;
 	if ((ptr + size < end) &&
 	    !__link_object(object_r, ptr + size, end - ptr - size,
-			   object->min_count, is_phys))
+			   object->min_count, objflags))
 		object_r = NULL;
 
 unlock:
@@ -879,11 +895,11 @@ static void paint_it(struct kmemleak_object *object, int color)
 	raw_spin_unlock_irqrestore(&object->lock, flags);
 }
 
-static void paint_ptr(unsigned long ptr, int color, bool is_phys)
+static void paint_ptr(unsigned long ptr, int color, unsigned int objflags)
 {
 	struct kmemleak_object *object;
 
-	object = __find_and_get_object(ptr, 0, is_phys);
+	object = __find_and_get_object(ptr, 0, objflags);
 	if (!object) {
 		kmemleak_warn("Trying to color unknown object at 0x%08lx as %s\n",
 			      ptr,
@@ -901,16 +917,16 @@ static void paint_ptr(unsigned long ptr, int color, bool is_phys)
  */
 static void make_gray_object(unsigned long ptr)
 {
-	paint_ptr(ptr, KMEMLEAK_GREY, false);
+	paint_ptr(ptr, KMEMLEAK_GREY, 0);
 }
 
 /*
  * Mark the object as black-colored so that it is ignored from scans and
  * reporting.
  */
-static void make_black_object(unsigned long ptr, bool is_phys)
+static void make_black_object(unsigned long ptr, unsigned int objflags)
 {
-	paint_ptr(ptr, KMEMLEAK_BLACK, is_phys);
+	paint_ptr(ptr, KMEMLEAK_BLACK, objflags);
 }
 
 /*
@@ -1046,8 +1062,6 @@ EXPORT_SYMBOL_GPL(kmemleak_alloc);
 void __ref kmemleak_alloc_percpu(const void __percpu *ptr, size_t size,
 				 gfp_t gfp)
 {
-	unsigned int cpu;
-
 	pr_debug("%s(0x%px, %zu)\n", __func__, ptr, size);
 
 	/*
@@ -1055,9 +1069,7 @@ void __ref kmemleak_alloc_percpu(const void __percpu *ptr, size_t size,
 	 * (min_count is set to 0).
 	 */
 	if (kmemleak_enabled && ptr && !IS_ERR(ptr))
-		for_each_possible_cpu(cpu)
-			create_object((unsigned long)per_cpu_ptr(ptr, cpu),
-				      size, 0, gfp);
+		create_object_percpu((unsigned long)ptr, size, 0, gfp);
 }
 EXPORT_SYMBOL_GPL(kmemleak_alloc_percpu);
 
@@ -1098,7 +1110,7 @@ void __ref kmemleak_free(const void *ptr)
 	pr_debug("%s(0x%px)\n", __func__, ptr);
 
 	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
-		delete_object_full((unsigned long)ptr);
+		delete_object_full((unsigned long)ptr, 0);
 }
 EXPORT_SYMBOL_GPL(kmemleak_free);
 
@@ -1116,7 +1128,7 @@ void __ref kmemleak_free_part(const void *ptr, size_t size)
 	pr_debug("%s(0x%px)\n", __func__, ptr);
 
 	if (kmemleak_enabled && ptr && !IS_ERR(ptr))
-		delete_object_part((unsigned long)ptr, size, false);
+		delete_object_part((unsigned long)ptr, size, 0);
 }
 EXPORT_SYMBOL_GPL(kmemleak_free_part);
 
@@ -1129,14 +1141,10 @@ EXPORT_SYMBOL_GPL(kmemleak_free_part);
  */
 void __ref kmemleak_free_percpu(const void __percpu *ptr)
 {
-	unsigned int cpu;
-
 	pr_debug("%s(0x%px)\n", __func__, ptr);
 
 	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
-		for_each_possible_cpu(cpu)
-			delete_object_full((unsigned long)per_cpu_ptr(ptr,
-								      cpu));
+		delete_object_full((unsigned long)ptr, OBJECT_PERCPU);
 }
 EXPORT_SYMBOL_GPL(kmemleak_free_percpu);
 
@@ -1204,7 +1212,7 @@ void __ref kmemleak_ignore(const void *ptr)
 	pr_debug("%s(0x%px)\n", __func__, ptr);
 
 	if (kmemleak_enabled && ptr && !IS_ERR(ptr))
-		make_black_object((unsigned long)ptr, false);
+		make_black_object((unsigned long)ptr, 0);
 }
 EXPORT_SYMBOL(kmemleak_ignore);
 
@@ -1278,7 +1286,7 @@ void __ref kmemleak_free_part_phys(phys_addr_t phys, size_t size)
 	pr_debug("%s(0x%px)\n", __func__, &phys);
 
 	if (kmemleak_enabled)
-		delete_object_part((unsigned long)phys, size, true);
+		delete_object_part((unsigned long)phys, size, OBJECT_PHYS);
 }
 EXPORT_SYMBOL(kmemleak_free_part_phys);
 
@@ -1292,7 +1300,7 @@ void __ref kmemleak_ignore_phys(phys_addr_t phys)
 	pr_debug("%s(0x%px)\n", __func__, &phys);
 
 	if (kmemleak_enabled)
-		make_black_object((unsigned long)phys, true);
+		make_black_object((unsigned long)phys, OBJECT_PHYS);
 }
 EXPORT_SYMBOL(kmemleak_ignore_phys);
 
@@ -1303,7 +1311,7 @@ static bool update_checksum(struct kmemleak_object *object)
 {
 	u32 old_csum = object->checksum;
 
-	if (WARN_ON_ONCE(object->flags & OBJECT_PHYS))
+	if (WARN_ON_ONCE(object->flags & (OBJECT_PHYS | OBJECT_PERCPU)))
 		return false;
 
 	kasan_disable_current();
@@ -1459,7 +1467,6 @@ static void scan_object(struct kmemleak_object *object)
 {
 	struct kmemleak_scan_area *area;
 	unsigned long flags;
-	void *obj_ptr;
 
 	/*
 	 * Once the object->lock is acquired, the corresponding memory block
@@ -1472,14 +1479,27 @@ static void scan_object(struct kmemleak_object *object)
 		/* already freed object */
 		goto out;
 
-	obj_ptr = object->flags & OBJECT_PHYS ?
-		  __va((phys_addr_t)object->pointer) :
-		  (void *)object->pointer;
+	if (object->flags & OBJECT_PERCPU) {
+		unsigned int cpu;
 
-	if (hlist_empty(&object->area_list) ||
+		for_each_possible_cpu(cpu) {
+			void *start = per_cpu_ptr((void __percpu *)object->pointer, cpu);
+			void *end = start + object->size;
+
+			scan_block(start, end, object);
+
+			raw_spin_unlock_irqrestore(&object->lock, flags);
+			cond_resched();
+			raw_spin_lock_irqsave(&object->lock, flags);
+			if (!(object->flags & OBJECT_ALLOCATED))
+				break;
+		}
+	} else if (hlist_empty(&object->area_list) ||
 	    object->flags & OBJECT_FULL_SCAN) {
-		void *start = obj_ptr;
-		void *end = obj_ptr + object->size;
+		void *start = object->flags & OBJECT_PHYS ?
+				__va((phys_addr_t)object->pointer) :
+				(void *)object->pointer;
+		void *end = start + object->size;
 		void *next;
 
 		do {
@@ -1494,11 +1514,12 @@ static void scan_object(struct kmemleak_object *object)
 			cond_resched();
 			raw_spin_lock_irqsave(&object->lock, flags);
 		} while (object->flags & OBJECT_ALLOCATED);
-	} else
+	} else {
 		hlist_for_each_entry(area, &object->area_list, node)
 			scan_block((void *)area->start,
 				   (void *)(area->start + area->size),
 				   object);
+	}
 out:
 	raw_spin_unlock_irqrestore(&object->lock, flags);
 }

-- 
Catalin

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