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

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Message-ID: <j7tcixbgyo5ongfv33d2h4dvmko7ygrwi5fww2nc2op54nmvz4@ofwy5jpcvcrn>
Date: Tue, 14 Oct 2025 15:40:51 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Hao Ge <hao.ge@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Harry Yoo <harry.yoo@...cle.com>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Hao Ge <gehao@...inos.cn>, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v3] slab: Add check for memcg_data != OBJEXTS_ALLOC_FAIL
 in folio_memcg_kmem

On Tue, Oct 14, 2025 at 10:58:25PM +0200, Vlastimil Babka wrote:
> On 10/14/25 22:14, Shakeel Butt wrote:
> > On Tue, Oct 14, 2025 at 09:12:43AM -0700, Suren Baghdasaryan wrote:
> >> On Tue, Oct 14, 2025 at 8:28 AM Hao Ge <hao.ge@...ux.dev> wrote:
> >> >
> >> > 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>
> >> 
> >> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> >> 
> >> nit: I think it would be helpful if the changelog explained why we
> >> need the additional check. We can have the same bit set in two
> >> different situations:
> >> 1. object extension vector allocation failure;
> >> 2. memcg_data pointing to a valid mem_cgroup.
> >> To distinguish between them, we need to check not only the bit itself
> >> but also the rest of this field. If the rest is NULL, we have case 1,
> >> otherwise case 2.
> > 
> > With Suren's suggestion, you can add:
> > 
> > Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> 
> Thanks, I added Suren's suggestion and pushed to slab/for-next-fixes:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/commit/?h=slab/for-next-fixes&id=711c435c89e59ee32bf8bb1c0d875a07931da5a8
> 
> Resisted the impulse to change the single VM_BUG_ON_FOLIO to
> VM_WARN_ON_ONCE_FOLIO because we're still going to do that systematically,
> right?

Oh is there some coordinated effort happening for this conversion?