On Mon, Jun 17, 2024 at 06:38:52PM +0200, Vlastimil Babka wrote:

On 6/17/24 6:33 PM, Jason A. Donenfeld wrote:

quoted

On Mon, Jun 17, 2024 at 6:30 PM Uladzislau Rezki [off-list ref] wrote:

quoted

Here if an "err" is less then "0" means there are still objects
whereas "is_destroyed" is set to "true" which is not correlated
with a comment:

"Destruction happens when no objects"

The comment is just poorly written. But the logic of the code is right.

quoted

quoted

 out_unlock:
      mutex_unlock(&slab_mutex);
      cpus_read_unlock();

diff --git a/mm/slub.c b/mm/slub.c
index 1373ac365a46..7db8fe90a323 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -4510,6 +4510,8 @@ void kmem_cache_free(struct kmem_cache *s, void *x)
              return;
      trace_kmem_cache_free(_RET_IP_, x, s);
      slab_free(s, virt_to_slab(x), x, _RET_IP_);
+     if (s->is_destroyed)
+             kmem_cache_destroy(s);
 }
 EXPORT_SYMBOL(kmem_cache_free);

@@ -5342,9 +5344,6 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
              if (!slab->inuse) {
                      remove_partial(n, slab);
                      list_add(&slab->slab_list, &discard);
-             } else {
-                     list_slab_objects(s, slab,
-                       "Objects remaining in %s on __kmem_cache_shutdown()");
              }
      }
      spin_unlock_irq(&n->list_lock);

Anyway it looks like it was not welcome to do it in the kmem_cache_free()
function due to performance reason.

"was not welcome" - Vlastimil mentioned *potential* performance
concerns before I posted this. I suspect he might have a different
view now, maybe?

Vlastimil, this is just checking a boolean (which could be
unlikely()'d), which should have pretty minimal overhead. Is that
alright with you?

Well I doubt we can just set and check it without any barriers? The
completion of the last pending kfree_rcu() might race with
kmem_cache_destroy() in a way that will leave the cache there forever, no?
And once we add barriers it becomes a perf issue?

Hm, yea you might be right about barriers being required. But actually,
might this point toward a larger problem with no matter what approach,
polling or event, is chosen? If the current rule is that
kmem_cache_free() must never race with kmem_cache_destroy(), because
users have always made diligent use of call_rcu()/rcu_barrier() and
such, but now we're going to let those race with each other - either by
my thing above or by polling - so we're potentially going to get in trouble
and need some barriers anyway. 

I think?

Jason