mm,page_owner: check for null stack_record before bumping its refcount

Patch series "page_owner: Fixup and cleanup".

This patchset consists of a fixup by an error that was reported by intel
robot, where it seems to be that by the time page_owner gets initialized,
stackdepot has already depleted its allocation space and returns
0-handles, turning that into null stack_records when trying to retrieve
the stack_record.  I was not able to reproduce that from the config
because it booted fine for me, but when setting e.g: dummy_handle to 0
artificially, I could see the same error that was reported.

The second patch is a cleanup that can also lead to a compilation warning.


This patch (of 2):

Although the retrieval of the stack_records for {dummy,failure}_handle
happen when page_owner gets initialized, there seems to be some situations
where stackdepot space has been already depleted by then, so we get
0-handles which make stack_records being NULL for those cases.

Be careful to 1) only bump stack_records refcount and 2) only access
stack_record fields if we actually have a non-null stack_record between
hands.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 4bedfb3 ("mm,page_owner: implement the tracking of the stacks count")
Signed-off-by: Oscar Salvador <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/page_owner.c

@@ -107,8 +107,10 @@ static __init void init_page_owner(void)
 	/* Initialize dummy and failure stacks and link them to stack_list */
 	dummy_stack.stack_record = __stack_depot_get_stack_record(dummy_handle);
 	failure_stack.stack_record = __stack_depot_get_stack_record(failure_handle);
-	refcount_set(&dummy_stack.stack_record->count, 1);
-	refcount_set(&failure_stack.stack_record->count, 1);
+	if (dummy_stack.stack_record)
+		refcount_set(&dummy_stack.stack_record->count, 1);
+	if (failure_stack.stack_record)
+		refcount_set(&failure_stack.stack_record->count, 1);
 	dummy_stack.next = &failure_stack;
 	stack_list = &dummy_stack;
 }
@@ -856,6 +858,9 @@ static int stack_print(struct seq_file *m, void *v)
 	unsigned long nr_entries;
 	struct stack_record *stack_record = stack->stack_record;
 
+	if (!stack->stack_record)
+		return 0;
+
 	nr_entries = stack_record->size;
 	entries = stack_record->entries;
 	stack_count = refcount_read(&stack_record->count) - 1;