linux-kernel - Re: [PATCH v3 32/35] codetag: debug: skip objext checking when it's for objext itself

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Date: Fri, 16 Feb 2024 19:39:56 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
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Subject: Re: [PATCH v3 32/35] codetag: debug: skip objext checking when it's
 for objext itself

On 2/12/24 22:39, Suren Baghdasaryan wrote:
> objext objects are created with __GFP_NO_OBJ_EXT flag and therefore have
> no corresponding objext themselves (otherwise we would get an infinite
> recursion). When freeing these objects their codetag will be empty and
> when CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled this will lead to false
> warnings. Introduce CODETAG_EMPTY special codetag value to mark
> allocations which intentionally lack codetag to avoid these warnings.
> Set objext codetags to CODETAG_EMPTY before freeing to indicate that
> the codetag is expected to be empty.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>  include/linux/alloc_tag.h | 26 ++++++++++++++++++++++++++
>  mm/slab.h                 | 25 +++++++++++++++++++++++++
>  mm/slab_common.c          |  1 +
>  mm/slub.c                 |  8 ++++++++
>  4 files changed, 60 insertions(+)
> 
> diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
> index 0a5973c4ad77..1f3207097b03 100644

..

> index c4bd0d5348cb..cf332a839bf4 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -567,6 +567,31 @@ static inline struct slabobj_ext *slab_obj_exts(struct slab *slab)
>  int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  			gfp_t gfp, bool new_slab);
>  
> +
> +#ifdef CONFIG_MEM_ALLOC_PROFILING_DEBUG
> +
> +static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
> +{
> +	struct slabobj_ext *slab_exts;
> +	struct slab *obj_exts_slab;
> +
> +	obj_exts_slab = virt_to_slab(obj_exts);
> +	slab_exts = slab_obj_exts(obj_exts_slab);
> +	if (slab_exts) {
> +		unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
> +						 obj_exts_slab, obj_exts);
> +		/* codetag should be NULL */
> +		WARN_ON(slab_exts[offs].ref.ct);
> +		set_codetag_empty(&slab_exts[offs].ref);
> +	}
> +}
> +
> +#else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> +
> +static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
> +
> +#endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> +

I assume with alloc_slab_obj_exts() moved to slub.c, mark_objexts_empty()
could move there too.

>  static inline bool need_slab_obj_ext(void)
>  {
>  #ifdef CONFIG_MEM_ALLOC_PROFILING
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 21b0b9e9cd9e..d5f75d04ced2 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -242,6 +242,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  		 * assign slabobj_exts in parallel. In this case the existing
>  		 * objcg vector should be reused.
>  		 */
> +		mark_objexts_empty(vec);
>  		kfree(vec);
>  		return 0;
>  	}
> diff --git a/mm/slub.c b/mm/slub.c
> index 4d480784942e..1136ff18b4fe 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1890,6 +1890,14 @@ static inline void free_slab_obj_exts(struct slab *slab)
>  	if (!obj_exts)
>  		return;
>  
> +	/*
> +	 * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
> +	 * corresponding extension will be NULL. alloc_tag_sub() will throw a
> +	 * warning if slab has extensions but the extension of an object is
> +	 * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
> +	 * the extension for obj_exts is expected to be NULL.
> +	 */
> +	mark_objexts_empty(obj_exts);
>  	kfree(obj_exts);
>  	slab->obj_exts = 0;
>  }