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

From: Uladzislau Rezki <[email-masked]>
Date: Mon, 17 Jun 2024 20:42:09 +0200

On Mon, Jun 17, 2024 at 07:23:36PM +0200, Vlastimil Babka wrote:
> On 6/17/24 6:12 PM, Paul E. McKenney wrote:
> > On Mon, Jun 17, 2024 at 05:10:50PM +0200, Vlastimil Babka wrote:
> >> On 6/13/24 2:22 PM, Jason A. Donenfeld wrote:
> >> > On Wed, Jun 12, 2024 at 08:38:02PM -0700, Paul E. McKenney wrote:
> >> >> o	Make the current kmem_cache_destroy() asynchronously wait for
> >> >> 	all memory to be returned, then complete the destruction.
> >> >> 	(This gets rid of a valuable debugging technique because
> >> >> 	in normal use, it is a bug to attempt to destroy a kmem_cache
> >> >> 	that has objects still allocated.)
> >> 
> >> This seems like the best option to me. As Jason already said, the debugging
> >> technique is not affected significantly, if the warning just occurs
> >> asynchronously later. The module can be already unloaded at that point, as
> >> the leak is never checked programatically anyway to control further
> >> execution, it's just a splat in dmesg.
> > 
> > Works for me!
> 
> Great. So this is how a prototype could look like, hopefully? The kunit test
> does generate the splat for me, which should be because the rcu_barrier() in
> the implementation (marked to be replaced with the real thing) is really
> insufficient. Note the test itself passes as this kind of error isn't wired
> up properly.
> 
> Another thing to resolve is the marked comment about kasan_shutdown() with
> potential kfree_rcu()'s in flight.
> 
> Also you need CONFIG_SLUB_DEBUG enabled otherwise node_nr_slabs() is a no-op
> and it might fail to notice the pending slabs. This will need to change.
> 
> ----8<----
> diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c
> index e6667a28c014..e3e4d0ca40b7 100644
> --- a/lib/slub_kunit.c
> +++ b/lib/slub_kunit.c
> @@ -5,6 +5,7 @@
>  #include <linux/slab.h>
>  #include <linux/module.h>
>  #include <linux/kernel.h>
> +#include <linux/rcupdate.h>
>  #include "../mm/slab.h"
>  
>  static struct kunit_resource resource;
> @@ -157,6 +158,26 @@ static void test_kmalloc_redzone_access(struct kunit *test)
>  	kmem_cache_destroy(s);
>  }
>  
> +struct test_kfree_rcu_struct {
> +	struct rcu_head rcu;
> +};
> +
> +static void test_kfree_rcu(struct kunit *test)
> +{
> +	struct kmem_cache *s = test_kmem_cache_create("TestSlub_kfree_rcu",
> +				sizeof(struct test_kfree_rcu_struct),
> +				SLAB_NO_MERGE);
> +	struct test_kfree_rcu_struct *p = kmem_cache_alloc(s, GFP_KERNEL);
> +
> +	kasan_disable_current();
> +
> +	KUNIT_EXPECT_EQ(test, 0, slab_errors);
> +
> +	kasan_enable_current();
> +	kfree_rcu(p, rcu);
> +	kmem_cache_destroy(s);
> +}
> +
>  static int test_init(struct kunit *test)
>  {
>  	slab_errors = 0;
> @@ -177,6 +198,7 @@ static struct kunit_case test_cases[] = {
>  
>  	KUNIT_CASE(test_clobber_redzone_free),
>  	KUNIT_CASE(test_kmalloc_redzone_access),
> +	KUNIT_CASE(test_kfree_rcu),
>  	{}
>  };
>  
> diff --git a/mm/slab.h b/mm/slab.h
> index b16e63191578..a0295600af92 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -277,6 +277,8 @@ struct kmem_cache {
>  	unsigned int red_left_pad;	/* Left redzone padding size */
>  	const char *name;		/* Name (only for display!) */
>  	struct list_head list;		/* List of slab caches */
> +	struct work_struct async_destroy_work;
> +
>  #ifdef CONFIG_SYSFS
>  	struct kobject kobj;		/* For sysfs */
>  #endif
> @@ -474,7 +476,7 @@ static inline bool is_kmalloc_cache(struct kmem_cache *s)
>  			      SLAB_NO_USER_FLAGS)
>  
>  bool __kmem_cache_empty(struct kmem_cache *);
> -int __kmem_cache_shutdown(struct kmem_cache *);
> +int __kmem_cache_shutdown(struct kmem_cache *, bool);
>  void __kmem_cache_release(struct kmem_cache *);
>  int __kmem_cache_shrink(struct kmem_cache *);
>  void slab_kmem_cache_release(struct kmem_cache *);
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 5b1f996bed06..c5c356d0235d 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -44,6 +44,8 @@ static LIST_HEAD(slab_caches_to_rcu_destroy);
>  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 void kmem_cache_kfree_rcu_destroy_workfn(struct work_struct *work);
> +
>  
>  /*
>   * Set of flags that will prevent slab merging
> @@ -234,6 +236,7 @@ static struct kmem_cache *create_cache(const char *name,
>  
>  	s->refcount = 1;
>  	list_add(&s->list, &slab_caches);
> +	INIT_WORK(&s->async_destroy_work, kmem_cache_kfree_rcu_destroy_workfn);
>  	return s;
>  
>  out_free_cache:
> @@ -449,12 +452,16 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>  	}
>  }
>  
> -static int shutdown_cache(struct kmem_cache *s)
> +static int shutdown_cache(struct kmem_cache *s, bool warn_inuse)
>  {
>  	/* free asan quarantined objects */
> +	/*
> +	 * XXX: is it ok to call this multiple times? and what happens with a
> +	 * kfree_rcu() in flight that finishes after or in parallel with this?
> +	 */
>  	kasan_cache_shutdown(s);
>  
> -	if (__kmem_cache_shutdown(s) != 0)
> +	if (__kmem_cache_shutdown(s, warn_inuse) != 0)
>  		return -EBUSY;
>  
>  	list_del(&s->list);
> @@ -477,6 +484,32 @@ void slab_kmem_cache_release(struct kmem_cache *s)
>  	kmem_cache_free(kmem_cache, s);
>  }
>  
> +static void kmem_cache_kfree_rcu_destroy_workfn(struct work_struct *work)
> +{
> +	struct kmem_cache *s;
> +	int err = -EBUSY;
> +	bool rcu_set;
> +
> +	s = container_of(work, struct kmem_cache, async_destroy_work);
> +
> +	// XXX use the real kmem_cache_free_barrier() or similar thing here
It implies that we need to introduce kfree_rcu_barrier(), a new API, which i
wanted to avoid initially. Since you do it asynchronous can we just repeat
and wait until it a cache is furry freed?

I am asking because inventing a new kfree_rcu_barrier() might not be so
straight forward.

--
Uladzislau Rezki