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

Harry Yoo
2025-10-23T07:50:44Z

On Thu, Oct 23, 2025 at 11:11:56AM +0800, Hao Ge wrote:
> 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@kylinos.cn>
> > >
> > > 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@kylinos.cn>
> > > ---
> > > 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.

Oh, just to be clear I was talking about the other warning:
pr_warn_once("%s, %s: Failed to create slab extension vector!", __func__, s->name);

> 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 yeah I see what you mean.

As you mentioned, if the process that failed to allocate the vector wins
cmpxchg(), later process that successfully allocate the vector would
call set_codetag_empty(), so no warning.

But if the process that allocates the vector wins cmpxchg(),
then it won't call set_codetag_empty(), so the process
that was trying to set OBJEXTS_ALLOC_FAIL now needs to set the tag.

> > But anyway, I think there is a better way to do this:

What do you think about the diff I suggested below, though?

> > 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;
> > }
> >
>

--
Cheers,
Harry / Hyeonggon