linux-kernel - Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL in folio_memcg

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Message-ID: <87y0pddw5m.fsf@linux.dev>
Date: Tue, 14 Oct 2025 14:13:25 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Hao Ge <hao.ge@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>,  Alexei Starovoitov <ast@...nel.org>,
  Andrew Morton <akpm@...ux-foundation.org>,  Johannes Weiner
 <hannes@...xchg.org>,  Shakeel Butt <shakeel.butt@...ux.dev>,  Michal
 Hocko <mhocko@...nel.org>,  Muchun Song <muchun.song@...ux.dev>,  Suren
 Baghdasaryan <surenb@...gle.com>,  Harry Yoo <harry.yoo@...cle.com>,
  cgroups@...r.kernel.org,  linux-mm@...ck.org,
  linux-kernel@...r.kernel.org,  Hao Ge <gehao@...inos.cn>
Subject: Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL
 in folio_memcg_kmem

Hao Ge <hao.ge@...ux.dev> writes:

> From: Hao Ge <gehao@...inos.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@...cle.com>
> Signed-off-by: Hao Ge <gehao@...inos.cn>
> ---
> v3: Simplify the solution, per Harry's suggestion in the v1 comments
>     Add Suggested-by: Harry Yoo <harry.yoo@...cle.com>

Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thanks!