Re: [PATCH v5] slab: reset obj_ext when it is not actually valid during freeing

[Suren Baghdasaryan]

On Wed, Oct 15, 2025 at 7:17 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
>
> From: Hao Ge <gehao@xxxxxxxxxx>
>
> If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> But we did not clear it when freeing the slab. Since OBJEXTS_ALLOC_FAIL and
> MEMCG_DATA_OBJEXTS currently share the same bit position, during the
> release of the associated folio, a VM_BUG_ON_FOLIO() check in
> folio_memcg_kmem() is triggered because it was mistakenly assumed that
> a valid folio->memcg_data was not cleared before freeing the folio.
>
> When freeing a slab, we clear slab->obj_exts and reset it to 0
> if the obj_ext array has been successfully allocated.
> So let's reset slab->obj_exts to 0 when freeing a slab if
> the obj_ext array allocated fail to allow them to be returned
> to the buddy system more smoothly.
>
> Signed-off-by: Hao Ge <gehao@xxxxxxxxxx>
> ---
> v5: Adopt the simpler solution proposed by Vlastimil;
>     Many thanks to him
> ---
>  mm/slub.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index b1f15598fbfd..2e4340c75be2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2170,8 +2170,16 @@ static inline void free_slab_obj_exts(struct slab *slab)
>         struct slabobj_ext *obj_exts;
>
>         obj_exts = slab_obj_exts(slab);
> -       if (!obj_exts)
> +       if (!obj_exts) {
> +               /*
> +                * If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> +                * In this case, we will end up here.
> +                * Therefore, we should clear the OBJEXTS_ALLOC_FAIL flag first when freeing a slab.
> +                * Then let's set it to 0 as below.
> +                */
> +               slab->obj_exts = 0;
>                 return;
> +       }

How about this instead:

static inline void free_slab_obj_exts(struct slab *slab)
{
        struct slabobj_ext *obj_exts;

        obj_exts = slab_obj_exts(slab);
+        /*
+         * Reset obj_exts to ensure all bits including OBJEXTS_ALLOC_FAIL
+         * are always cleared.
+         */
+        slab->obj_exts = 0;
        if (!obj_exts)
                return;

        /*
         * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
         * corresponding extension will be NULL. alloc_tag_sub() will throw a
         * warning if slab has extensions but the extension of an object is
         * NULL, therefore replace NULL with CODETAG_EMPTY to indicate that
         * the extension for obj_exts is expected to be NULL.
         */
        mark_objexts_empty(obj_exts);
        kfree(obj_exts);
-        slab->obj_exts = 0;
}

>
>         /*
>          * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
> --
> 2.25.1
>