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From: Oscar Salvador <osalvador@suse.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org,
	linux-mm@kvack.org,
	Michal Hocko <mhocko@suse.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Marco Elver <elver@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	Oscar Salvador <osalvador@suse.de>,
	kernel test robot <oliver.sang@intel.com>
Subject: [PATCH 1/2] mm,page_owner: Check for null stack_record before bumping its refcount
Date: Wed,  6 Mar 2024 13:32:16 +0100
Message-ID: <20240306123217.29774-2-osalvador@suse.de>
In-Reply-To: <20240306123217.29774-1-osalvador@suse.de>
References: <20240306123217.29774-1-osalvador@suse.de>
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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.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202403051032.e2f865a-lkp@intel.com
Fixes: 4bedfb314bdd ("mm,page_owner: implement the tracking of the stacks count")
Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
 mm/page_owner.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 033e349f6479..7163a1c44ccf 100644
--- 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;
-- 
2.44.0