linux-kernel - Re: [PATCH v3] huge_mm.h: disallow is_huge_zero

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Message-ID: <81B36E34-BDD0-4A66-925C-CCC809248992@nvidia.com>
Date: Thu, 28 Aug 2025 21:25:26 -0400
From: Zi Yan <ziy@...dia.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org,
 shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
 bhe@...hat.com, chrisl@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] huge_mm.h: disallow is_huge_zero_folio(NULL)

On 28 Aug 2025, at 4:48, Max Kellermann wrote:

> Calling is_huge_zero_folio(NULL) should not be legal - it makes no
> sense, and a different (theoretical) implementation may dereference
> the pointer.  But currently, lacking any explicit documentation, this
> call is possible.
>
> But if somebody really passes NULL, the function should not return
> true - this isn't the huge zero folio after all!  However, if the
> `huge_zero_folio` hasn't been allocated yet, it's NULL, and
> is_huge_zero_folio(NULL) just happens to return true, which is a lie.
>
> This weird side effect prevented me from reproducing a kernel crash
> that occurred when the elements of a folio_batch were NULL - since
> folios_put_refs() skips huge zero folios, this sometimes causes a
> crash, but sometimes does not.  For debugging, it is better to reveal
> such bugs reliably and not hide them behind random preconditions like
> "has the huge zero folio already been created?"
>
> To improve detection of such bugs, David Hildenbrand suggested adding
> a VM_WARN_ON_ONCE().
>
> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
> ---
> v1->v2: using VM_WARN_ON_ONCE() instead of checking huge_zero_folio
> v2->v3: use "!" to check NULL; removed the #include
> ---
>  include/linux/huge_mm.h | 2 ++
>  1 file changed, 2 insertions(+)
>

LGTM. Reviewed-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi