linux-kernel - Re: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj

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Message-ID: <aPbs_cu261GnQgcu@hyeyoo>
Date: Tue, 21 Oct 2025 11:16:29 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Hao Ge <hao.ge@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Shakeel Butt <shakeel.butt@...ux.dev>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hao Ge <gehao@...inos.cn>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] slab: Avoid race on slab->obj_exts in
 alloc_slab_obj_exts

On Tue, Oct 21, 2025 at 09:03:53AM +0800, Hao Ge wrote:
> From: Hao Ge <gehao@...inos.cn>
> 
> If two competing threads enter alloc_slab_obj_exts() and one of them
> fails to allocate the object extension vector, it might override the
> valid slab->obj_exts allocated by the other thread with
> OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and
> expects a valid pointer to dereference a NULL pointer later on.
> 
> Update slab->obj_exts atomically using cmpxchg() to avoid
> slab->obj_exts overrides by racing threads.
> 
> Thanks for Vlastimil and Suren's help with debugging.
> 
> Fixes: f7381b911640 ("slab: mark slab->obj_exts allocation failures unconditionally")
> Cc: <stable@...r.kernel.org>
> Suggested-by: Suren Baghdasaryan <surenb@...gle.com>
> Signed-off-by: Hao Ge <gehao@...inos.cn>
> ---
> v3: According to Suren's suggestion, simplify the commit message and the code comments.
>     Thanks for Suren.
> 
> v2: Incorporate handling for the scenario where, if mark_failed_objexts_alloc wins the race,
>     the other process (that previously succeeded in allocation) will lose the race, based on Suren's suggestion.
>     Add Suggested-by: Suren Baghdasaryan <surenb@...gle.com>
> ---

Looks good to me, thanks for fixing this!

Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

-- 
Cheers,
Harry / Hyeonggon