[PATCH v8 3/5] mm,page_owner: Display all stacks and their count

[Oscar Salvador <osalvador@suse.de>]

This patch adds a new directory called 'page_owner_stacks' under
/sys/kernel/debug/, with a file called 'show_stacks' in it.
Reading from that file will show all stacks that were added by page_owner
followed by their counting, giving us a clear overview of stack <-> count
relationship.

E.g:

  prep_new_page+0xa9/0x120
  get_page_from_freelist+0x801/0x2210
  __alloc_pages+0x18b/0x350
  alloc_pages_mpol+0x91/0x1f0
  folio_alloc+0x14/0x50
  filemap_alloc_folio+0xb2/0x100
  __filemap_get_folio+0x14a/0x490
  ext4_write_begin+0xbd/0x4b0 [ext4]
  generic_perform_write+0xc1/0x1e0
  ext4_buffered_write_iter+0x68/0xe0 [ext4]
  ext4_file_write_iter+0x70/0x740 [ext4]
  vfs_write+0x33d/0x420
  ksys_write+0xa5/0xe0
  do_syscall_64+0x80/0x160
  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 stack_count: 4578

The seq stack_{start,next} functions will iterate through the list
stack_list in order to print all stacks.

Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
 mm/page_owner.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 98 insertions(+), 1 deletion(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 7d1b3f75cef3..3e4b7cd7c8f8 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -84,7 +84,12 @@ static void add_stack_record_to_list(struct stack_record *stack_record)
 			stack_list = stack;
 		} else {
 			stack->next = stack_list;
-			stack_list = stack;
+			/* This pairs with smp_load_acquire() from function
+			 * stack_start(). This guarantees that stack_start()
+			 * will see an updated stack_list before starting to
+			 * traverse the list.
+			 */
+			smp_store_release(&stack_list, stack);
 		}
 		spin_unlock_irqrestore(&stack_list_lock, flags);
 	}
@@ -792,8 +797,97 @@ static const struct file_operations proc_page_owner_operations = {
 	.llseek		= lseek_page_owner,
 };
 
+static void *stack_start(struct seq_file *m, loff_t *ppos)
+{
+	struct stack *stack;
+
+	if (*ppos == -1UL)
+		return NULL;
+
+	if (!*ppos) {
+		/*
+		 * This pairs with smp_store_release() from function
+		 * add_stack_record_to_list(), so we get a consistent
+		 * value of stack_list.
+		 */
+		stack = smp_load_acquire(&stack_list);
+	} else {
+		stack = m->private;
+		stack = stack->next;
+	}
+
+	m->private = stack;
+
+	return stack;
+}
+
+static void *stack_next(struct seq_file *m, void *v, loff_t *ppos)
+{
+	struct stack *stack = v;
+
+	stack = stack->next;
+	*ppos = stack ? *ppos + 1 : -1UL;
+	m->private = stack;
+
+	return stack;
+}
+
+static int stack_print(struct seq_file *m, void *v)
+{
+	char *buf;
+	int ret = 0;
+	struct stack *stack = v;
+	struct stack_record *stack_record = stack->stack_record;
+
+	if (!stack_record->size || stack_record->size < 0 ||
+	    refcount_read(&stack_record->count) < 2)
+		return 0;
+
+	buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
+
+	ret += stack_trace_snprint(buf, PAGE_SIZE, stack_record->entries,
+				   stack_record->size, 0);
+	if (!ret)
+		goto out;
+
+	scnprintf(buf + ret, PAGE_SIZE - ret, "stack_count: %d\n\n",
+		  refcount_read(&stack_record->count));
+
+	seq_printf(m, buf);
+	seq_puts(m, "\n\n");
+out:
+	kfree(buf);
+
+	return 0;
+}
+
+static void stack_stop(struct seq_file *m, void *v)
+{
+}
+
+static const struct seq_operations page_owner_stack_op = {
+	.start	= stack_start,
+	.next	= stack_next,
+	.stop	= stack_stop,
+	.show	= stack_print
+};
+
+static int page_owner_stack_open(struct inode *inode, struct file *file)
+{
+	return seq_open_private(file, &page_owner_stack_op, 0);
+}
+
+static const struct file_operations page_owner_stack_operations = {
+	.open		= page_owner_stack_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release,
+};
+
 static int __init pageowner_init(void)
 {
+	struct dentry *dir;
+
 	if (!static_branch_unlikely(&page_owner_inited)) {
 		pr_info("page_owner is disabled\n");
 		return 0;
@@ -801,6 +895,9 @@ static int __init pageowner_init(void)
 
 	debugfs_create_file("page_owner", 0400, NULL, NULL,
 			    &proc_page_owner_operations);
+	dir = debugfs_create_dir("page_owner_stacks", NULL);
+	debugfs_create_file("show_stacks", 0400, dir, NULL,
+			    &page_owner_stack_operations);
 
 	return 0;
 }
-- 
2.43.0