[PATCH v2 1/6] lib/stackdepot: allow requesting early initialization dynamically

[Vlastimil Babka]

In a later patch we want to add stackdepot support for object owner
tracking in slub caches, which is enabled by slub_debug boot parameter.
This creates a bootstrap problem as some caches are created early in
boot when slab_is_available() is false and thus stack_depot_init()
tries to use memblock. But, as reported by Hyeonggon Yoo [1] we are
already beyond memblock_free_all(). Ideally memblock allocation should
fail, yet it succeeds, but later the system crashes, which is a
separately handled issue.

To resolve this boostrap issue in a robust way, this patch adds another
way to request stack_depot_early_init(), which happens at a well-defined
point of time. In addition to build-time CONFIG_STACKDEPOT_ALWAYS_INIT,
code that's e.g. processing boot parmeters (which happens early enough)
can set a new variable stack_depot_want_early_init as true.

In this patch we also convert page_owner to this approach. While it
doesn't have the bootstrap issue as slub, it's also a functionality
enabled by a boot param and can thus request stack_depot_early_init()
with memblock allocation instead of later initialization with
kvmalloc().

[1] https://lore.kernel.org/all/YhnUcqyeMgCrWZbd@xxxxxxxxxxxxxxxxxxx-northeast-1.compute.internal/

Reported-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 include/linux/stackdepot.h | 16 ++++++++++++++--
 lib/stackdepot.c           |  2 ++
 mm/page_owner.c            |  9 ++++++---
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
index 17f992fe6355..1217ba2b636e 100644
--- a/include/linux/stackdepot.h
+++ b/include/linux/stackdepot.h
@@ -15,6 +15,8 @@
 
 typedef u32 depot_stack_handle_t;
 
+extern bool stack_depot_want_early_init;
+
 depot_stack_handle_t __stack_depot_save(unsigned long *entries,
 					unsigned int nr_entries,
 					gfp_t gfp_flags, bool can_alloc);
@@ -26,11 +28,21 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
  * The alternative is to select STACKDEPOT_ALWAYS_INIT to have stack depot
  * enabled as part of mm_init(), for subsystems where it's known at compile time
  * that stack depot will be used.
+ *
+ * Another alternative is to set stack_depot_want_early_init as true, when the
+ * decision to use stack depot is taken e.g. when evaluating kernel boot
+ * parameters, which precedes the call to stack_depot_want_early_init().
  */
 int stack_depot_init(void);
 
-#ifdef CONFIG_STACKDEPOT_ALWAYS_INIT
-static inline int stack_depot_early_init(void)	{ return stack_depot_init(); }
+#ifdef CONFIG_STACKDEPOT
+static inline int stack_depot_early_init(void)
+{
+	if (IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT)
+	    || stack_depot_want_early_init)
+		return stack_depot_init();
+	return 0;
+}
 #else
 static inline int stack_depot_early_init(void)	{ return 0; }
 #endif
diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index bf5ba9af0500..02e2b5fcbf3b 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -66,6 +66,8 @@ struct stack_record {
 	unsigned long entries[];	/* Variable-sized array of entries. */
 };
 
+bool stack_depot_want_early_init = false;
+
 static void *stack_slabs[STACK_ALLOC_MAX_SLABS];
 
 static int depot_index;
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 99e360df9465..40dce2b81d13 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -42,7 +42,12 @@ static void init_early_allocated_pages(void);
 
 static int __init early_page_owner_param(char *buf)
 {
-	return kstrtobool(buf, &page_owner_enabled);
+	int ret = kstrtobool(buf, &page_owner_enabled);
+
+	if (page_owner_enabled)
+		stack_depot_want_early_init = true;
+
+	return ret;
 }
 early_param("page_owner", early_page_owner_param);
 
@@ -80,8 +85,6 @@ static __init void init_page_owner(void)
 	if (!page_owner_enabled)
 		return;
 
-	stack_depot_init();
-
 	register_dummy_stack();
 	register_failure_stack();
 	register_early_stack();
-- 
2.35.1