Re: [PATCH v2] mm: slub: call WARN() instead of pr_err on slab_fix.

[Vlastimil Babka <vbabka@suse.cz>]

On 2/5/25 01:46, 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 better to use WARN() instead of pr_err to catch
> errors at the point of issue because WARN() could trigger panic for
> system debugging when panic_on_warn is enabled. WARN() should be
> called prior to fixing the value because when a panic is triggered by WARN(),
> it allows us to check corrupted data.
> 
> Changes in v2:
> - Replace direct calling with BUG_ON with the use of WARN in slab_fix.
> 
> Signed-off-by: Hyesoo Yu <hyesoo.yu@samsung.com>

Hi and thanks for the patch,

I wonder if it would be better not to change slab_fix() but rather
slab_err() and object_err(). It wouldn't then require to rearrange the fixup
code. Also I think some error reporting paths don't go through slab_fix()
and we still would like them to become a WARN too.

Basically it would mean the last line in slab_err() would be a WARN and we'd
drop the dump_stack() as that's redundant. Same in object_err() (no
dump_stack() there). It would be a bit noisier as a result, but hopefully
acceptable. The slab specific debugging info would still be printed before
the WARN hits (and potentially results in a panic) so anyone investigating
the crash dump would have that information.

Hm but I see some places print stuff after slab_err(). slab_pad_check() and
list_slab_objects(). We could create slab_err_start() and slab_err_end() for
those, and slab_err() would just call both at once.

> ---
>  mm/slub.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1f50129dcfb3..ea956cb4b8be 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1043,7 +1043,7 @@ static void slab_fix(struct kmem_cache *s, char *fmt, ...)
>  	va_start(args, fmt);
>  	vaf.fmt = fmt;
>  	vaf.va = &args;
> -	pr_err("FIX %s: %pV\n", s->name, &vaf);
> +	WARN(1, "FIX %s: %pV\n", s->name, &vaf);
>  	va_end(args);
>  }
>  
> @@ -1106,8 +1106,8 @@ static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
>  	if ((s->flags & SLAB_CONSISTENCY_CHECKS) &&
>  	    !check_valid_pointer(s, slab, nextfree) && freelist) {
>  		object_err(s, slab, *freelist, "Freechain corrupt");
> -		*freelist = NULL;
>  		slab_fix(s, "Isolate corrupted freechain");
> +		*freelist = NULL;
>  		return true;
>  	}
>  
> @@ -1445,9 +1445,9 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
>  				set_freepointer(s, object, NULL);
>  			} else {
>  				slab_err(s, slab, "Freepointer corrupt");
> +				slab_fix(s, "Freelist cleared");
>  				slab->freelist = NULL;
>  				slab->inuse = slab->objects;
> -				slab_fix(s, "Freelist cleared");
>  				return 0;
>  			}
>  			break;
> @@ -1464,14 +1464,14 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
>  	if (slab->objects != max_objects) {
>  		slab_err(s, slab, "Wrong number of objects. Found %d but should be %d",
>  			 slab->objects, max_objects);
> -		slab->objects = max_objects;
>  		slab_fix(s, "Number of objects adjusted");
> +		slab->objects = max_objects;
>  	}
>  	if (slab->inuse != slab->objects - nr) {
>  		slab_err(s, slab, "Wrong object count. Counter is %d but counted were %d",
>  			 slab->inuse, slab->objects - nr);
> -		slab->inuse = slab->objects - nr;
>  		slab_fix(s, "Object count adjusted");
> +		slab->inuse = slab->objects - nr;
>  	}
>  	return search == NULL;
>  }