Re: [PATCH slab/for-next-fixes v3 4/4] mm/slab: prevent unbounded recursion in free path with new kmalloc type

[Vlastimil Babka (SUSE)]

On 7/13/26 16:28, Harry Yoo (Oracle) wrote:
> @@ -2168,14 +2132,20 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  	unsigned long new_exts;
>  	unsigned long old_exts;
>  	struct slabobj_ext *vec;
> -	size_t sz;
> +	size_t sz = sizeof(struct slabobj_ext) * slab->objects;
>  
>  	gfp &= ~OBJCGS_CLEAR_MASK;
> -	/* Prevent recursive extension vector allocation */
> -	alloc_flags |= SLAB_ALLOC_NO_RECURSE;
> -	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
> +	/*
> +	 * In most cases, obj_exts arrays are allocated from normal kmalloc.
> +	 * However, normal kmalloc caches must allocate them from
> +	 * KMALLOC_NO_OBJ_EXT caches to prevent recursion.
> +	 */
> +	if (is_kmalloc_normal(s)) {
> +		VM_WARN_ON_ONCE(!need_kmalloc_no_objext());

As discussed off-list, this can false positively trigger due to a race with
shutdown_mem_profiling() so I'll remove it.

> +		alloc_flags |= SLAB_ALLOC_NO_OBJ_EXT;
> +	}
>  
> -	sz = obj_exts_alloc_size(s, slab, gfp);
> +	alloc_flags &= ~SLAB_ALLOC_NEW_SLAB;
>  
>  	/* This will use kmalloc_nolock() if alloc_flags say so */
>  	vec = kmalloc_flags(sz, gfp | __GFP_ZERO, alloc_flags, slab_nid(slab));