We touch slab->freelist and slab->inuse before checking the slab pointer
is actually sane. Do that validation first, which will be safer. We can
thus also remove the check from alloc_debug_processing().

This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
alloc_single_from_partial() is only called for caches with debugging
enabled so it's acceptable.

In alloc_single_from_new_slab() we just created the struct slab and call
alloc_debug_processing() to mainly set up redzones, tracking etc, while
not really expecting the consistency checks to fail. Thus don't validate
it there.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 12ad42f3d2e066b02340f2c30a85422583af3c5d..e5b53d1debddd3fe0f941f579a1043a5b976e50b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -821,6 +821,8 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
 	return *(unsigned int *)p;
 }
 
+#ifdef CONFIG_SLUB_DEBUG
+
 /*
  * For debugging context when we want to check if the struct slab pointer
  * appears to be valid.
@@ -830,7 +832,6 @@ static inline bool validate_slab_ptr(struct slab *slab)
 	return PageSlab(slab_page(slab));
 }
 
-#ifdef CONFIG_SLUB_DEBUG
 static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
 static DEFINE_SPINLOCK(object_map_lock);
 
@@ -1651,11 +1652,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
 			struct slab *slab, void *object, int orig_size)
 {
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
-		if (!validate_slab_ptr(slab)) {
-			slab_err(s, slab, "Not a valid slab page");
-			return false;
-		}
-
 		if (!alloc_consistency_checks(s, slab, object))
 			goto bad;
 	}
@@ -2825,13 +2821,21 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
 
 	lockdep_assert_held(&n->list_lock);
 
+#ifdef SLUB_DEBUG
+	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!validate_slab_ptr(slab)) {
+			slab_err(s, slab, "Not a valid slab page");
+			return NULL;
+		}
+	}
+#endif
+
 	object = slab->freelist;
 	slab->freelist = get_freepointer(s, object);
 	slab->inuse++;
 
 	if (!alloc_debug_processing(s, slab, object, orig_size)) {
-		if (validate_slab_ptr(slab))
-			remove_partial(n, slab);
+		remove_partial(n, slab);
 		return NULL;
 	}
 

-- 
2.51.0

On Mon, Sep 15, 2025 at 03:55:12PM +0200, Vlastimil Babka wrote:
> We touch slab->freelist and slab->inuse before checking the slab pointer
> is actually sane. Do that validation first, which will be safer. We can
> thus also remove the check from alloc_debug_processing().
> 
> This adds a new "s->flags & SLAB_CONSISTENCY_CHECKS" test but
> alloc_single_from_partial() is only called for caches with debugging
> enabled so it's acceptable.
> 
> In alloc_single_from_new_slab() we just created the struct slab and call
> alloc_debug_processing() to mainly set up redzones, tracking etc, while
> not really expecting the consistency checks to fail. Thus don't validate
> it there.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 12ad42f3d2e066b02340f2c30a85422583af3c5d..e5b53d1debddd3fe0f941f579a1043a5b976e50b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -821,6 +821,8 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
>  	return *(unsigned int *)p;
>  }
>  
> +#ifdef CONFIG_SLUB_DEBUG
> +
>  /*
>   * For debugging context when we want to check if the struct slab pointer
>   * appears to be valid.
> @@ -830,7 +832,6 @@ static inline bool validate_slab_ptr(struct slab *slab)
>  	return PageSlab(slab_page(slab));
>  }
>  
> -#ifdef CONFIG_SLUB_DEBUG
>  static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
>  static DEFINE_SPINLOCK(object_map_lock);
>  
> @@ -1651,11 +1652,6 @@ static noinline bool alloc_debug_processing(struct kmem_cache *s,
>  			struct slab *slab, void *object, int orig_size)
>  {
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> -		if (!validate_slab_ptr(slab)) {
> -			slab_err(s, slab, "Not a valid slab page");
> -			return false;
> -		}
> -
>  		if (!alloc_consistency_checks(s, slab, object))
>  			goto bad;
>  	}
> @@ -2825,13 +2821,21 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>  
>  	lockdep_assert_held(&n->list_lock);
>  
> +#ifdef SLUB_DEBUG

I'm sure you meant CONFIG_SLUB_DEBUG ;)

With that adjusted, looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

> +	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
> +		if (!validate_slab_ptr(slab)) {
> +			slab_err(s, slab, "Not a valid slab page");
> +			return NULL;
> +		}
> +	}
> +#endif
> +
>  	object = slab->freelist;
>  	slab->freelist = get_freepointer(s, object);
>  	slab->inuse++;
>  
>  	if (!alloc_debug_processing(s, slab, object, orig_size)) {
> -		if (validate_slab_ptr(slab))
> -			remove_partial(n, slab);
> +		remove_partial(n, slab);
>  		return NULL;
>  	}

-- 
Cheers,
Harry / Hyeonggon

On 9/15/25 16:25, Harry Yoo wrote:
> On Mon, Sep 15, 2025 at 03:55:12PM +0200, Vlastimil Babka wrote:
>> @@ -2825,13 +2821,21 @@ static void *alloc_single_from_partial(struct kmem_cache *s,
>>  
>>  	lockdep_assert_held(&n->list_lock);
>>  
>> +#ifdef SLUB_DEBUG
> 
> I'm sure you meant CONFIG_SLUB_DEBUG ;)
> 
> With that adjusted, looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

Doh yes, thanks a lot :)
>> +	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>> +		if (!validate_slab_ptr(slab)) {
>> +			slab_err(s, slab, "Not a valid slab page");
>> +			return NULL;
>> +		}
>> +	}
>> +#endif
>> +
>>  	object = slab->freelist;
>>  	slab->freelist = get_freepointer(s, object);
>>  	slab->inuse++;
>>  
>>  	if (!alloc_debug_processing(s, slab, object, orig_size)) {
>> -		if (validate_slab_ptr(slab))
>> -			remove_partial(n, slab);
>> +		remove_partial(n, slab);
>>  		return NULL;
>>  	}
>