[RFC] SLUB: list_slab_objects() looks like a miss-merge

[Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>]

Hi,

I stumbled over the following:

| static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
| {
…
|         unsigned long *map = NULL;
| 
| #ifdef CONFIG_SLUB_DEBUG
|         map = bitmap_alloc(oo_objects(s->max), GFP_KERNEL);
| #endif
…
|                 } else {
|                         list_slab_objects(s, page,
|                           "Objects remaining in %s on __kmem_cache_shutdown()",
|                           map);
|                 }
|         }
…
| #ifdef CONFIG_SLUB_DEBUG
|         bitmap_free(map);
| #endif
…
| }


so map gets allocated, passed to list_slab_objects() and then freed at
end. Haven't notices where `map' is set but maybe it is just a buffer
used in list_slab_objects().
And then there is this:

| static void list_slab_objects(struct kmem_cache *s, struct page *page,
|                               const char *text, unsigned long *map)
| {
| #ifdef CONFIG_SLUB_DEBUG
|         void *addr = page_address(page);
|         void *p;
| 
|         if (!map)
|                 return;

No map, return. Okay.

|         slab_err(s, page, text, s->name);
|         slab_lock(page);
| 
|         map = get_map(s, page);

and here `map' gets overwritten, correct? But it gets initialized. And it also
acquires `object_map_lock'.

|         for_each_object(p, s, addr, page->objects) {
| 
|                 if (!test_bit(slab_index(p, s, addr), map)) {
|                         pr_err("INFO: Object 0x%p @offset=%tu\n", p, p - addr);
|                         print_tracking(s, p);
|                 }
|         }

and here I would expect to unlock `object_map_lock'.

|         slab_unlock(page);
| #endif
| }   

Is this a miss-merge of some kind? I would revert commit
   aa456c7aebb14 ("slub: remove kmalloc under list_lock from list_slab_objects() V2")

by doing this:

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
deleted file mode 100644
index e69de29bb2d1d..0000000000000
diff --git a/mm/slub.c b/mm/slub.c
index b8f798b50d44d..72195cafbb503 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3766,15 +3766,13 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
 }
 
 static void list_slab_objects(struct kmem_cache *s, struct page *page,
-			      const char *text, unsigned long *map)
+			      const char *text)
 {
 #ifdef CONFIG_SLUB_DEBUG
 	void *addr = page_address(page);
+	unsigned long *map;
 	void *p;
 
-	if (!map)
-		return;
-
 	slab_err(s, page, text, s->name);
 	slab_lock(page);
 
@@ -3786,6 +3784,7 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
 			print_tracking(s, p);
 		}
 	}
+	put_map(map);
 	slab_unlock(page);
 #endif
 }
@@ -3799,11 +3798,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
 {
 	LIST_HEAD(discard);
 	struct page *page, *h;
-	unsigned long *map = NULL;
-
-#ifdef CONFIG_SLUB_DEBUG
-	map = bitmap_alloc(oo_objects(s->max), GFP_KERNEL);
-#endif
 
 	BUG_ON(irqs_disabled());
 	spin_lock_irq(&n->list_lock);
@@ -3813,16 +3807,11 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
 			list_add(&page->slab_list, &discard);
 		} else {
 			list_slab_objects(s, page,
-			  "Objects remaining in %s on __kmem_cache_shutdown()",
-			  map);
+			  "Objects remaining in %s on __kmem_cache_shutdown()");
 		}
 	}
 	spin_unlock_irq(&n->list_lock);
 
-#ifdef CONFIG_SLUB_DEBUG
-	bitmap_free(map);
-#endif
-
 	list_for_each_entry_safe(page, h, &discard, slab_list)
 		discard_slab(s, page);
 }


Is there something I'm missing? This is completely untested of course.

Sebastian