Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Oct 14, 2025 at 09:12:43AM -0700, Suren Baghdasaryan wrote:
> On Tue, Oct 14, 2025 at 8:28 AM Hao Ge <hao.ge@linux.dev> wrote:
> >
> > From: Hao Ge <gehao@kylinos.cn>
> >
> > Since OBJEXTS_ALLOC_FAIL and MEMCG_DATA_OBJEXTS currently share
> > the same bit position, we cannot determine whether memcg_data still
> > points to the slabobj_ext vector simply by checking
> > folio->memcg_data & MEMCG_DATA_OBJEXTS.
> >
> > If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> > and during the release of the associated folio, the BUG check is triggered
> > because it was mistakenly assumed that a valid folio->memcg_data
> > was not cleared before freeing the folio.
> >
> > So let's check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg_kmem.
> >
> > Fixes: 7612833192d5 ("slab: Reuse first bit for OBJEXTS_ALLOC_FAIL")
> > Suggested-by: Harry Yoo <harry.yoo@oracle.com>
> > Signed-off-by: Hao Ge <gehao@kylinos.cn>
> 
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> 
> nit: I think it would be helpful if the changelog explained why we
> need the additional check. We can have the same bit set in two
> different situations:
> 1. object extension vector allocation failure;
> 2. memcg_data pointing to a valid mem_cgroup.
> To distinguish between them, we need to check not only the bit itself
> but also the rest of this field. If the rest is NULL, we have case 1,
> otherwise case 2.

With Suren's suggestion, you can add:

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>