Re: [PATCH V2 2/2] mm/slab: drop the OBJEXTS_NOSPIN_ALLOC flag from enum objext_flags

[Hao Li <hao.li@linux.dev>]

On Tue, Feb 10, 2026 at 01:46:42PM +0900, Harry Yoo wrote:
> OBJEXTS_NOSPIN_ALLOC was used to remember whether a slabobj_ext vector
> was allocated via kmalloc_nolock(), so that free_slab_obj_exts() could
> call kfree_nolock() instead of kfree().
> 
> Now that kfree() supports freeing kmalloc_nolock() objects, this flag is
> no longer needed. Instead, pass the allow_spin parameter down to
> free_slab_obj_exts() to determine whether kfree_nolock() or kfree()
> should be called in the free path, and free one bit in
> enum objext_flags.
> 
> Acked-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/memcontrol.h |  3 +--
>  mm/slub.c                  | 18 ++++++++----------
>  2 files changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0651865a4564..bb789ec4a2a2 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -359,8 +359,7 @@ enum objext_flags {
>  	 * MEMCG_DATA_OBJEXTS.
>  	 */
>  	OBJEXTS_ALLOC_FAIL = __OBJEXTS_ALLOC_FAIL,
> -	/* slabobj_ext vector allocated with kmalloc_nolock() */
> -	OBJEXTS_NOSPIN_ALLOC = __FIRST_OBJEXT_FLAG,
> +	__OBJEXTS_FLAG_UNUSED = __FIRST_OBJEXT_FLAG,
>  	/* the next bit after the last actual flag */
>  	__NR_OBJEXTS_FLAGS  = (__FIRST_OBJEXT_FLAG << 1),
>  };
> diff --git a/mm/slub.c b/mm/slub.c
> index 63b03fd62ca7..a73a80b33ff9 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2189,8 +2189,6 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  			virt_to_slab(vec)->slab_cache == s);
>  
>  	new_exts = (unsigned long)vec;
> -	if (unlikely(!allow_spin))
> -		new_exts |= OBJEXTS_NOSPIN_ALLOC;
>  #ifdef CONFIG_MEMCG
>  	new_exts |= MEMCG_DATA_OBJEXTS;
>  #endif
> @@ -2228,7 +2226,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  	return 0;
>  }
>  
> -static inline void free_slab_obj_exts(struct slab *slab)
> +static inline void free_slab_obj_exts(struct slab *slab, bool allow_spin)
>  {
>  	struct slabobj_ext *obj_exts;
>  
> @@ -2256,10 +2254,10 @@ static inline void free_slab_obj_exts(struct slab *slab)
>  	 * the extension for obj_exts is expected to be NULL.
>  	 */
>  	mark_objexts_empty(obj_exts);
> -	if (unlikely(READ_ONCE(slab->obj_exts) & OBJEXTS_NOSPIN_ALLOC))
> -		kfree_nolock(obj_exts);
> -	else
> +	if (allow_spin)
>  		kfree(obj_exts);
> +	else
> +		kfree_nolock(obj_exts);

Looks good to me.

One small observation from my side: at first glance I briefly wondered if we
could ever allocate in an allow_spin=true context but free in an
allow_spin=false context, potentially resulting in a kmalloc() -> kfree_nolock()
mismatch.

After taking a closer look, the only case where "free_new_slab_nolock() -> ...
-> free_slab_obj_exts()" runs with allow_spin=false is along the "___slab_alloc
-> alloc_single_from_new_slab()/alloc_from_new_slab()" path. In that scenario,
alloc_slab_obj_exts() is also called under allow_spin=false and uses
kmalloc_nolock(), so the allocation/free paths remain consistent. So "kmalloc()
-> kfree_nolock()" case can't happen, and the code looks solid to me!

Reviewed-by: Hao Li <hao.li@linux.dev>

>  	slab->obj_exts = 0;
>  }
>  
> @@ -2323,7 +2321,7 @@ static int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  	return 0;
>  }
>  
> -static inline void free_slab_obj_exts(struct slab *slab)
> +static inline void free_slab_obj_exts(struct slab *slab, bool allow_spin)
>  {
>  }
>  
> @@ -3387,14 +3385,14 @@ static __always_inline void account_slab(struct slab *slab, int order,
>  }
>  
>  static __always_inline void unaccount_slab(struct slab *slab, int order,
> -					   struct kmem_cache *s)
> +					   struct kmem_cache *s, bool allow_spin)
>  {
>  	/*
>  	 * The slab object extensions should now be freed regardless of
>  	 * whether mem_alloc_profiling_enabled() or not because profiling
>  	 * might have been disabled after slab->obj_exts got allocated.
>  	 */
> -	free_slab_obj_exts(slab);
> +	free_slab_obj_exts(slab, allow_spin);
>  
>  	mod_node_page_state(slab_pgdat(slab), cache_vmstat_idx(s),
>  			    -(PAGE_SIZE << order));
> @@ -3498,7 +3496,7 @@ static void __free_slab(struct kmem_cache *s, struct slab *slab, bool allow_spin
>  	page->mapping = NULL;
>  	__ClearPageSlab(page);
>  	mm_account_reclaimed_pages(pages);
> -	unaccount_slab(slab, order, s);
> +	unaccount_slab(slab, order, s, allow_spin);
>  	if (allow_spin)
>  		free_frozen_pages(page, order);
>  	else
> -- 
> 2.43.0
>