linux-kernel - Re: [PATCH v4 2/2] mm: slub: call WARN() when the slab detect an error

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Message-ID: <50cb2f6c-f1cd-4aac-9336-e2c88f6314bf@suse.cz>
Date: Thu, 27 Feb 2025 15:38:48 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Hyesoo Yu <hyesoo.yu@...sung.com>
Cc: janghyuck.kim@...sung.com, harry.yoo@...cle.com,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm: slub: call WARN() when the slab detect an
 error

On 2/26/25 09:12, Hyesoo Yu wrote:
> If a slab object is corrupted or an error occurs in its internal
> value, continuing after restoration may cause other side effects.
> At this point, it is difficult to debug because the problem occurred
> in the past. It is useful to use WARN() to catch errors at the point
> of issue because WARN() could trigger panic for system debugging when
> panic_on_warn is enabled. WARN() is added where to detect the error
> on slab_err and object_err.
> 
> It makes sense to only do the WARN() after printing the logs. slab_err
> is splited to __slab_err that calls the WARN() and it is called after
> printing logs.
> 
> Changes in v4:
> - Remove WARN() in kmem_cache_destroy to remove redundant warning.
> 
> Changes in v3:
> - move the WARN from slab_fix to slab_err, object_err and check_obj to
> use WARN on all error reporting paths.
> 
> Changes in v2:
> - Replace direct calling with BUG_ON with the use of WARN in slab_fix.
> 
> Signed-off-by: Hyesoo Yu <hyesoo.yu@...sung.com>

As Harry said. I'll remove that locally.

> ---
>  mm/slab_common.c |  3 ---
>  mm/slub.c        | 31 +++++++++++++++++++------------
>  2 files changed, 19 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 477fa471da18..d13f4ffe252b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -517,9 +517,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	kasan_cache_shutdown(s);
>  
>  	err = __kmem_cache_shutdown(s);
> -	if (!slab_in_kunit_test())
> -		WARN(err, "%s %s: Slab cache still has objects when called from %pS",
> -		     __func__, s->name, (void *)_RET_IP_);

I think I'll keep this one, because the more detailed warning via
list_slab_objects() is only enabled with CONFIG_SLUB_DEBUG.
If it's not enabled, the kmem_cache_destroy() failure rather should not be
silent.
So slab_in_kunit_test() would also stay.

>  		} else {
> -			list_slab_objects(s, slab,
> -			  "Objects remaining in %s on __kmem_cache_shutdown()");
> +			list_slab_objects(s, slab);

I tried to extract slab_bug() and __slab_err() from list_slab_objects() but
they were also only available with CONFIG_SLUB_DEBUG.

Perhaps we can improve that, but as a follow-up cleanup so we don't hold
this up further.

>  		}
>  	}
>  	spin_unlock_irq(&n->list_lock);