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

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Oct 15, 2025 at 06:44:38PM +0200, Vlastimil Babka wrote:
> On 10/15/25 18:29, Suren Baghdasaryan wrote:
> > On Wed, Oct 15, 2025 at 7:17 AM Hao Ge <hao.ge@linux.dev> wrote:
> >>
> >> From: Hao Ge <gehao@kylinos.cn>
> >>
> >> 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@kylinos.cn>
> >> ---
> >> 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?

You're not wrong!

> ----8<----
> From 8151384e5baf34db5812ed51e2e463796ab6e973 Mon Sep 17 00:00:00 2001
> From: Hao Ge <gehao@kylinos.cn>
> 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.

nit: maybe this can be massaged as "a VM_BUG_ON_FOLIO() check in folio_memcg_kmem()
is triggered because the OBJEXTS_ALLOC_FAIL flag was not cleared,
causing it to be interpreted as a kmem folio (non-slab) with
MEMCG_OBJEXTS_DATA flag set, which is invalid because MEMCG_OBJEXTS_DATA
is supposed to be set only on slabs."

> 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@linux.dev/
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry / Hyeonggon

>  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