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

[Hao Ge]

Hi Harry


On 2025/10/23 10:24, Harry Yoo wrote:
> On Thu, Oct 23, 2025 at 09:21:17AM +0800, Hao Ge wrote:
> > From: Hao Ge <gehao@xxxxxxxxxx>
> >
> > If two competing threads enter alloc_slab_obj_exts(), and the
> > thread that failed to allocate the object extension vector exits
> > after the one that succeeded, it will 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 alloc_slab_obj_exts fails.
> >
> > Signed-off-by: Hao Ge <gehao@xxxxxxxxxx>
> > ---
> >   mm/slub.c | 9 ++++++---
> >   1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index d4403341c9df..42276f0cc920 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2227,9 +2227,12 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
> >   	slab = virt_to_slab(p);
> >   	if (!slab_obj_exts(slab) &&
> >   	    alloc_slab_obj_exts(slab, s, flags, false)) {
> > -		pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
> > -			     __func__, s->name);
> > -		return NULL;
> > +		/* Recheck if a racing thread has successfully allocated slab->obj_exts. */
> > +		if (!slab_obj_exts(slab)) {
> > +			pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
> > +				     __func__, s->name);
> > +			return NULL;
> > +		}
> >   	}
> Maybe this patch is a bit paranoid... since if mark_failed_objexts_alloc()
> win cmpxchg() and then someone else allocates the object extension vector,
> the warning will still be printed anyway.


The process that successfully allocates slab_exts will call 
handle_failed_objexts_alloc, setting ref->ct = CODETAG_EMPTY

to prevent the warning from being triggered.


> But anyway, I think there is a better way to do this:
>
> diff --git a/mm/slub.c b/mm/slub.c
> index dd4c85ea1038..d08d7580349d 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 true; }
>   static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
>   			struct slabobj_ext *vec, unsigned int objects) {}
>   
> @@ -2125,7 +2125,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>   	}
>   	if (!vec) {
>   		/* Mark vectors which failed to allocate */
> -		mark_failed_objexts_alloc(slab);
> +		if (!mark_failed_objexts_alloc(slab) &&
> +			slab_obj_exts(slab))
> +			return 0;
>   
>   		return -ENOMEM;
>   	}