slub: Update frozen slabs documentations in the source

The current updated scheme (which this series implemented) is:
 - node partial slabs: PG_Workingset && !frozen
 - cpu partial slabs: !PG_Workingset && !frozen
 - cpu slabs: !PG_Workingset && frozen
 - full slabs: !PG_Workingset && !frozen

The most important change is that "frozen" bit is not set for the
cpu partial slabs anymore, __slab_free() will grab node list_lock
then check by !PG_Workingset that it's not on a node partial list.

And the "frozen" bit is still kept for the cpu slabs for performance,
since we don't need to grab node list_lock to check whether the
PG_Workingset is set or not if the "frozen" bit is set in __slab_free().

Update related documentations and comments in the source.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Christoph Lameter (Ampere) <cl@linux.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: Chengming Zhou

mm/slub.c

@@ -76,13 +76,28 @@
  *
  *   Frozen slabs
  *
- *   If a slab is frozen then it is exempt from list management. It is not
- *   on any list except per cpu partial list. The processor that froze the
+ *   If a slab is frozen then it is exempt from list management. It is
+ *   the cpu slab which is actively allocated from by the processor that
+ *   froze it and it is not on any list. The processor that froze the
  *   slab is the one who can perform list operations on the slab. Other
  *   processors may put objects onto the freelist but the processor that
  *   froze the slab is the only one that can retrieve the objects from the
  *   slab's freelist.
  *
+ *   CPU partial slabs
+ *
+ *   The partially empty slabs cached on the CPU partial list are used
+ *   for performance reasons, which speeds up the allocation process.
+ *   These slabs are not frozen, but are also exempt from list management,
+ *   by clearing the PG_workingset flag when moving out of the node
+ *   partial list. Please see __slab_free() for more details.
+ *
+ *   To sum up, the current scheme is:
+ *   - node partial slab: PG_Workingset && !frozen
+ *   - cpu partial slab: !PG_Workingset && !frozen
+ *   - cpu slab: !PG_Workingset && frozen
+ *   - full slab: !PG_Workingset && !frozen
+ *
  *   list_lock
  *
  *   The list_lock protects the partial and full list on each node and
@@ -2617,8 +2632,7 @@ static void put_partials_cpu(struct kmem_cache *s,
 }
 
 /*
- * Put a slab that was just frozen (in __slab_free|get_partial_node) into a
- * partial slab slot if available.
+ * Put a slab into a partial slab slot if available.
  *
  * If we did not find a slot then simply move all the partials to the
  * per node partial list.