linux-kernel - Re: [PATCH v2] slab: Fix obj_ext is mistakenly considered NULL due to race condition

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Message-ID: <CAJuCfpFBmmP5_44mYkae=XDb41Z_PyjSJk5oh3F8urc=fELkgw@mail.gmail.com>
Date: Thu, 23 Oct 2025 09:33:24 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Hao Ge <hao.ge@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH v2] slab: Fix obj_ext is mistakenly considered NULL due to
 race condition

On Thu, Oct 23, 2025 at 7:34 AM Hao Ge <hao.ge@...ux.dev> wrote:
>
> From: Hao Ge <gehao@...inos.cn>
>
> If two competing threads enter alloc_slab_obj_exts(), if the process
> that allocates the vector wins cmpxchg(), and the other thread mistakenly
> assume slab->obj_ext is still empty due to its own allocation failure. This
> will then trigger warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG
> checks in the subsequent free path.
>
> Therefore, let's add an additional check when the process that allocates
> the vector loses the cmpxchg()
>
> Suggested-by: Harry Yoo <harry.yoo@...cle.com>
> Signed-off-by: Hao Ge <gehao@...inos.cn>

Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

> ---
> v2: Revise the solution according to Harry's suggestion.
>     Add Suggested-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  mm/slub.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index d4403341c9df..d7bfec6c0171 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2052,9 +2052,9 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
>         }
>  }
>
> -static inline void mark_failed_objexts_alloc(struct slab *slab)
> +static inline bool mark_failed_objexts_alloc(struct slab *slab)
>  {
> -       cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
> +       return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0;
>  }
>
>  static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> @@ -2076,7 +2076,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>  #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>
>  static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
> -static inline void mark_failed_objexts_alloc(struct slab *slab) {}
> +static inline bool mark_failed_objexts_alloc(struct slab *slab) { return false; }
>  static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>                         struct slabobj_ext *vec, unsigned int objects) {}
>
> @@ -2124,8 +2124,14 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>                                    slab_nid(slab));
>         }
>         if (!vec) {
> -               /* Mark vectors which failed to allocate */
> -               mark_failed_objexts_alloc(slab);
> +               /*
> +                * Try to mark vectors which failed to allocate
> +                * If this operation fails, there may be a racing process
> +                * that has already completed the allocation.
> +                */
> +               if (!mark_failed_objexts_alloc(slab) &&
> +                   slab_obj_exts(slab))
> +                       return 0;
>
>                 return -ENOMEM;
>         }
> --
> 2.25.1
>