For monitoring the memory usage per stack trace, it is more efficient to
read _just_ the handle number of the stack traces _without_ stack traces
themselves, and their number of base pages.

The stack traces are required only at the time to associate memory usage
of a handle number with its stack trace (eg, see top-consuming handles).
Before that, it is sufficient to have just the handle number, as it can
be matched with a stack trace later (possible with the previous patch).

This patch adds the option 'print_stack' option (enabled by default) to
print stack traces in 'show_stacks'.

Testing:

  - Enable handle numbers:
  # echo 1 >/sys/kernel/debug/page_owner_stacks/print_handle

  - With stacks (default):
  # cat /sys/kernel/debug/page_owner_stacks/show_stacks > f1

  - Without stacks (new option):
  # echo 0 >/sys/kernel/debug/page_owner_stacks/print_stack
  # cat /sys/kernel/debug/page_owner_stacks/show_stacks > f2

  - Differences:

  # cat f1
   get_page_from_freelist+0x14ab/0x16a0
   ...
   do_syscall_64+0xa4/0x290
  handle: 15728651
  nr_base_pages: 2

   get_page_from_freelist+0x14ab/0x16a0
   ...
   do_sys_openat2+0x8a/0xe0
  handle: 8388619
  nr_base_pages: 16

  ...

  # cat f2
  handle: 15728651
  nr_base_pages: 2

  handle: 8388619
  nr_base_pages: 16

  ...

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
---
 mm/page_owner.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 420426749239..25221676676d 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -888,6 +888,7 @@ static void *stack_next(struct seq_file *m, void *v, loff_t *ppos)
 
 static unsigned long page_owner_pages_threshold;
 static bool page_owner_print_handle;
+static bool page_owner_print_stack = true;
 
 static int stack_print(struct seq_file *m, void *v)
 {
@@ -900,15 +901,17 @@ static int stack_print(struct seq_file *m, void *v)
 	if (!stack->stack_record)
 		return 0;
 
-	nr_entries = stack_record->size;
-	entries = stack_record->entries;
 	nr_base_pages = refcount_read(&stack_record->count) - 1;
 
 	if (nr_base_pages < 1 || nr_base_pages < page_owner_pages_threshold)
 		return 0;
 
-	for (i = 0; i < nr_entries; i++)
-		seq_printf(m, " %pS\n", (void *)entries[i]);
+	if (page_owner_print_stack) {
+		nr_entries = stack_record->size;
+		entries = stack_record->entries;
+		for (i = 0; i < nr_entries; i++)
+			seq_printf(m, " %pS\n", (void *)entries[i]);
+	}
 	if (page_owner_print_handle)
 		seq_printf(m, "handle: %d\n", stack_record->handle.handle);
 	seq_printf(m, "nr_base_pages: %d\n\n", nr_base_pages);
@@ -973,6 +976,8 @@ static int __init pageowner_init(void)
 			    &proc_page_owner_threshold);
 	debugfs_create_bool("print_handle", 0600, dir,
 			    &page_owner_print_handle);
+	debugfs_create_bool("print_stack", 0600, dir,
+			    &page_owner_print_stack);
 
 	return 0;
 }
-- 
2.48.1