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

[Vlastimil Babka]

On 10/15/25 18:29, Suren Baghdasaryan wrote:
> 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;

You have an older base, check current mainline, we evaluate slab->obj_exts
later in the function

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