Re: [PATCH] mm/slab_common: use WARN() if cache still has objects on destroy

[Vlastimil Babka <vbabka@suse.cz>]

On 11/2/21 18:07, Marco Elver wrote:
> Calling kmem_cache_destroy() while the cache still has objects allocated
> is a kernel bug, and will usually result in the entire cache being
> leaked. While the message in kmem_cache_destroy() resembles a warning,
> it is currently not implemented using a real WARN().
> 
> This is problematic for infrastructure testing the kernel, all of which
> rely on the specific format of WARN()s to pick up on bugs.
> 
> Some 13 years ago this used to be a simple WARN_ON() in slub, but
> d629d8195793 ("slub: improve kmem_cache_destroy() error message")
> changed it into an open-coded warning to avoid confusion with a bug in
> slub itself.
> 
> Instead, turn the open-coded warning into a real WARN() with the message
> preserved, so that test systems can actually identify these issues, and
> we get all the other benefits of using a normal WARN(). The warning
> message is extended with "when called from <caller-ip>" to make it even
> clearer where the fault lies.
> 
> For most configurations this is only a cosmetic change, however, note
> that WARN() here will now also respect panic_on_warn.
> 
> Signed-off-by: Marco Elver <elver@google.com>

Makes sense.

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/slab_common.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index ec2bb0beed75..0155a3042203 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -497,8 +497,6 @@ void slab_kmem_cache_release(struct kmem_cache *s)
>  
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
> -	int err;
> -
>  	if (unlikely(!s))
>  		return;
>  
> @@ -509,12 +507,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	if (s->refcount)
>  		goto out_unlock;
>  
> -	err = shutdown_cache(s);
> -	if (err) {
> -		pr_err("%s %s: Slab cache still has objects\n",
> -		       __func__, s->name);
> -		dump_stack();
> -	}
> +	WARN(shutdown_cache(s),
> +	     "%s %s: Slab cache still has objects when called from %pS",
> +	     __func__, s->name, (void *)_RET_IP_);
>  out_unlock:
>  	mutex_unlock(&slab_mutex);
>  	cpus_read_unlock();
>