Re: [PATCH 1/1] mm/slab: fix 'variable obj_exts set but not used' warning

[Vlastimil Babka <vbabka@suse.cz>]

On 6/15/24 12:59 AM, Suren Baghdasaryan wrote:
> slab_post_alloc_hook() uses prepare_slab_obj_exts_hook() to obtain
> slabobj_ext object. Currently the only user of slabobj_ext object in
> this path is memory allocation profiling, therefore when it's not enabled
> this object is not needed. This also generates a warning when compiling
> with CONFIG_MEM_ALLOC_PROFILING=n. Move the code under this configuration
> to fix the warning. If more slabobj_ext users appear in the future, the
> code will have to be changed back to call prepare_slab_obj_exts_hook().
> 
> Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202406150444.F6neSaiy-lkp@intel.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

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

But it seems to me we could remove the whole #ifdef if current->alloc_tag
(which doesn't exist with !MEM_ALLOC_PROFILING) had an access helper, or
there was a alloc_tag_add_current() variant?

> ---
>  mm/slub.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1373ac365a46..4927edec6a8c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3902,7 +3902,6 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  			  unsigned int orig_size)
>  {
>  	unsigned int zero_size = s->object_size;
> -	struct slabobj_ext *obj_exts;
>  	bool kasan_init = init;
>  	size_t i;
>  	gfp_t init_flags = flags & gfp_allowed_mask;
> @@ -3945,9 +3944,11 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  		kmemleak_alloc_recursive(p[i], s->object_size, 1,
>  					 s->flags, init_flags);
>  		kmsan_slab_alloc(s, p[i], init_flags);
> +#ifdef CONFIG_MEM_ALLOC_PROFILING
>  		if (need_slab_obj_ext()) {
> +			struct slabobj_ext *obj_exts;
> +
>  			obj_exts = prepare_slab_obj_exts_hook(s, flags, p[i]);
> -#ifdef CONFIG_MEM_ALLOC_PROFILING
>  			/*
>  			 * Currently obj_exts is used only for allocation profiling.
>  			 * If other users appear then mem_alloc_profiling_enabled()
> @@ -3955,8 +3956,8 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  			 */
>  			if (likely(obj_exts))
>  				alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
> -#endif
>  		}
> +#endif
>  	}
>  
>  	return memcg_slab_post_alloc_hook(s, lru, flags, size, p);
> 
> base-commit: c286c21ff94252f778515b21b6bebe749454a852