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/20240306123217.29774-1-osalvador@suse.de
Link: https://lkml.kernel.org/r/20240306123217.29774-2-osalvador@suse.de
Fixes: 4bedfb314bdd ("mm,page_owner: implement the tracking of the stacks count")
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202403051032.e2f865a-lkp@intel.com
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Marco Elver <elver@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 033e349..7163a1c 100644 (file)
--- a/mm/page_owner.c
+++ b/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;