Subject: Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

From: Uladzislau Rezki <[email-masked]>
Date: Fri, 14 Jun 2024 14:35:33 +0200

On Thu, Jun 13, 2024 at 11:13:52AM -0700, Paul E. McKenney wrote:
> On Thu, Jun 13, 2024 at 07:58:17PM +0200, Uladzislau Rezki wrote:
> > On Thu, Jun 13, 2024 at 10:45:59AM -0700, Paul E. McKenney wrote:
> > > On Thu, Jun 13, 2024 at 07:38:59PM +0200, Uladzislau Rezki wrote:
> > > > On Thu, Jun 13, 2024 at 08:06:30AM -0700, Paul E. McKenney wrote:
> > > > > On Thu, Jun 13, 2024 at 03:06:54PM +0200, Uladzislau Rezki wrote:
> > > > > > On Thu, Jun 13, 2024 at 05:47:08AM -0700, Paul E. McKenney wrote:
> > > > > > > On Thu, Jun 13, 2024 at 01:58:59PM +0200, Jason A. Donenfeld wrote:
> > > > > > > > On Wed, Jun 12, 2024 at 03:37:55PM -0700, Paul E. McKenney wrote:
> > > > > > > > > On Wed, Jun 12, 2024 at 02:33:05PM -0700, Jakub Kicinski wrote:
> > > > > > > > > > On Sun,  9 Jun 2024 10:27:12 +0200 Julia Lawall wrote:
> > > > > > > > > > > Since SLOB was removed, it is not necessary to use call_rcu
> > > > > > > > > > > when the callback only performs kmem_cache_free. Use
> > > > > > > > > > > kfree_rcu() directly.
> > > > > > > > > > > 
> > > > > > > > > > > The changes were done using the following Coccinelle semantic patch.
> > > > > > > > > > > This semantic patch is designed to ignore cases where the callback
> > > > > > > > > > > function is used in another way.
> > > > > > > > > > 
> > > > > > > > > > How does the discussion on:
> > > > > > > > > >   [PATCH] Revert "batman-adv: prefer kfree_rcu() over call_rcu() with free-only callbacks"
> > > > > > > > > >   https://lore.kernel.org/all/[email-masked]/
> > > > > > > > > > reflect on this series? IIUC we should hold off..
> > > > > > > > > 
> > > > > > > > > We do need to hold off for the ones in kernel modules (such as 07/14)
> > > > > > > > > where the kmem_cache is destroyed during module unload.
> > > > > > > > > 
> > > > > > > > > OK, I might as well go through them...
> > > > > > > > > 
> > > > > > > > > [PATCH 01/14] wireguard: allowedips: replace call_rcu by kfree_rcu for simple kmem_cache_free callback
> > > > > > > > > 	Needs to wait, see wg_allowedips_slab_uninit().
> > > > > > > > 
> > > > > > > > Also, notably, this patch needs additionally:
> > > > > > > > 
> > > > > > > > diff --git a/drivers/net/wireguard/allowedips.c b/drivers/net/wireguard/allowedips.c
> > > > > > > > index e4e1638fce1b..c95f6937c3f1 100644
> > > > > > > > --- a/drivers/net/wireguard/allowedips.c
> > > > > > > > +++ b/drivers/net/wireguard/allowedips.c
> > > > > > > > @@ -377,7 +377,6 @@ int __init wg_allowedips_slab_init(void)
> > > > > > > > 
> > > > > > > >  void wg_allowedips_slab_uninit(void)
> > > > > > > >  {
> > > > > > > > -	rcu_barrier();
> > > > > > > >  	kmem_cache_destroy(node_cache);
> > > > > > > >  }
> > > > > > > > 
> > > > > > > > Once kmem_cache_destroy has been fixed to be deferrable.
> > > > > > > > 
> > > > > > > > I assume the other patches are similar -- an rcu_barrier() can be
> > > > > > > > removed. So some manual meddling of these might be in order.
> > > > > > > 
> > > > > > > Assuming that the deferrable kmem_cache_destroy() is the option chosen,
> > > > > > > agreed.
> > > > > > >
> > > > > > <snip>
> > > > > > void kmem_cache_destroy(struct kmem_cache *s)
> > > > > > {
> > > > > > 	int err = -EBUSY;
> > > > > > 	bool rcu_set;
> > > > > > 
> > > > > > 	if (unlikely(!s) || !kasan_check_byte(s))
> > > > > > 		return;
> > > > > > 
> > > > > > 	cpus_read_lock();
> > > > > > 	mutex_lock(&slab_mutex);
> > > > > > 
> > > > > > 	rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
> > > > > > 
> > > > > > 	s->refcount--;
> > > > > > 	if (s->refcount)
> > > > > > 		goto out_unlock;
> > > > > > 
> > > > > > 	err = shutdown_cache(s);
> > > > > > 	WARN(err, "%s %s: Slab cache still has objects when called from %pS",
> > > > > > 	     __func__, s->name, (void *)_RET_IP_);
> > > > > > ...
> > > > > > 	cpus_read_unlock();
> > > > > > 	if (!err && !rcu_set)
> > > > > > 		kmem_cache_release(s);
> > > > > > }
> > > > > > <snip>
> > > > > > 
> > > > > > so we have SLAB_TYPESAFE_BY_RCU flag that defers freeing slab-pages
> > > > > > and a cache by a grace period. Similar flag can be added, like
> > > > > > SLAB_DESTROY_ONCE_FULLY_FREED, in this case a worker rearm itself
> > > > > > if there are still objects which should be freed.
> > > > > > 
> > > > > > Any thoughts here?
> > > > > 
> > > > > Wouldn't we also need some additional code to later check for all objects
> > > > > being freed to the slab, whether or not that code is  initiated from
> > > > > kmem_cache_destroy()?
> > > > >
> > > > Same away as SLAB_TYPESAFE_BY_RCU is handled from the kmem_cache_destroy() function.
> > > > It checks that flag and if it is true and extra worker is scheduled to perform a
> > > > deferred(instead of right away) destroy after rcu_barrier() finishes.
> > > 
> > > Like this?
> > > 
> > > 	SLAB_DESTROY_ONCE_FULLY_FREED
> > > 
> > > 	Instead of adding a new kmem_cache_destroy_rcu()
> > > 	or kmem_cache_destroy_wait() API member, instead add a
> > > 	SLAB_DESTROY_ONCE_FULLY_FREED flag that can be passed to the
> > > 	existing kmem_cache_destroy() function.  Use of this flag would
> > > 	suppress any warnings that would otherwise be issued if there
> > > 	was still slab memory yet to be freed, and it would also spawn
> > > 	workqueues (or timers or whatever) to do any needed cleanup work.
> > > 
> > >
> > The flag is passed as all others during creating a cache:
> > 
> >   slab = kmem_cache_create(name, size, ..., SLAB_DESTROY_ONCE_FULLY_FREED | OTHER_FLAGS, NULL);
> > 
> > the rest description is correct to me.
> 
> Good catch, fixed, thank you!
> 
And here we go with prototype(untested):

<snip>
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 7247e217e21b..700b8a909f8a 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -59,6 +59,7 @@ enum _slab_flag_bits {
 #ifdef CONFIG_SLAB_OBJ_EXT
 	_SLAB_NO_OBJ_EXT,
 #endif
+	_SLAB_DEFER_DESTROY,
 	_SLAB_FLAGS_LAST_BIT
 };
 
@@ -139,6 +140,7 @@ enum _slab_flag_bits {
  */
 /* Defer freeing slabs to RCU */
 #define SLAB_TYPESAFE_BY_RCU	__SLAB_FLAG_BIT(_SLAB_TYPESAFE_BY_RCU)
+#define SLAB_DEFER_DESTROY __SLAB_FLAG_BIT(_SLAB_DEFER_DESTROY)
 /* Trace allocations and frees */
 #define SLAB_TRACE		__SLAB_FLAG_BIT(_SLAB_TRACE)
 
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1560a1546bb1..99458a0197b5 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -45,6 +45,11 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work);
 static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
 		    slab_caches_to_rcu_destroy_workfn);
 
+static LIST_HEAD(slab_caches_defer_destroy);
+static void slab_caches_defer_destroy_workfn(struct work_struct *work);
+static DECLARE_DELAYED_WORK(slab_caches_defer_destroy_work,
+	slab_caches_defer_destroy_workfn);
+
 /*
  * Set of flags that will prevent slab merging
  */
@@ -448,6 +453,31 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
 	}
 }
 
+static void
+slab_caches_defer_destroy_workfn(struct work_struct *work)
+{
+	struct kmem_cache *s, *s2;
+
+	mutex_lock(&slab_mutex);
+	list_for_each_entry_safe(s, s2, &slab_caches_defer_destroy, list) {
+		if (__kmem_cache_empty(s)) {
+			/* free asan quarantined objects */
+			kasan_cache_shutdown(s);
+			(void) __kmem_cache_shutdown(s);
+
+			list_del(&s->list);
+
+			debugfs_slab_release(s);
+			kfence_shutdown_cache(s);
+			kmem_cache_release(s);
+		}
+	}
+	mutex_unlock(&slab_mutex);
+
+	if (!list_empty(&slab_caches_defer_destroy))
+		schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
+}
+
 static int shutdown_cache(struct kmem_cache *s)
 {
 	/* free asan quarantined objects */
@@ -493,6 +523,13 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	if (s->refcount)
 		goto out_unlock;
 
+	/* Should a destroy process be deferred? */
+	if (s->flags & SLAB_DEFER_DESTROY) {
+		list_move_tail(&s->list, &slab_caches_defer_destroy);
+		schedule_delayed_work(&slab_caches_defer_destroy_work, HZ);
+		goto out_unlock;
+	}
+
 	err = shutdown_cache(s);
 	WARN(err, "%s %s: Slab cache still has objects when called from %pS",
 	     __func__, s->name, (void *)_RET_IP_);
<snip>

Thanks!

--
Uladzislau Rezki