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

[Suren Baghdasaryan]

On Wed, Oct 15, 2025 at 9:44 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> 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
>
> I've massaged the commit log and comments a bit and also realized that
> AFAICS we're actually fixing an issue that predates 7612833192d5 ("slab:
> Reuse first bit for OBJEXTS_ALLOC_FAIL"). Am I wrong?
>
> ----8<----
> From 8151384e5baf34db5812ed51e2e463796ab6e973 Mon Sep 17 00:00:00 2001
> From: Hao Ge <gehao@xxxxxxxxxx>
> Date: Wed, 15 Oct 2025 22:16:42 +0800
> Subject: [PATCH] slab: reset slab->obj_ext when freeing and it is
>  OBJEXTS_ALLOC_FAIL
>
> If obj_exts allocation failed, slab->obj_exts is set to OBJEXTS_ALLOC_FAIL,
> But we do 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.
>
> Another problem that predates sharing the OBJEXTS_ALLOC_FAIL and
> MEMCG_DATA_OBJEXTS bits is that on configurations with
> is_check_pages_enabled(), the non-cleared bit in page->memcg_data will
> trigger a free_page_is_bad() failure "page still charged to cgroup"
>
> When freeing a slab, we clear slab->obj_exts if the obj_ext array has
> been successfully allocated. So let's clear it also when the allocation
> has failed.
>
> Fixes: 09c46563ff6d ("codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations")
> Link: https://lore.kernel.org/all/20251015141642.700170-1-hao.ge@xxxxxxxxx/
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Hao Ge <gehao@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/slub.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 13ae4491136a..a8fcc7e6f25a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2170,8 +2170,15 @@ 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 end up here and should
> +                * clear the flag.
> +                */
> +               slab->obj_exts = 0;
>                 return;
> +       }
>
>         /*
>          * obj_exts was created with __GFP_NO_OBJ_EXT flag, therefore its
> --
> 2.51.0
>
>