Re: [PATCH 1/2] huge_mm.h: is_huge_zero_folio(NULL) should return false

[Zi Yan]

On 26 Aug 2025, at 19:16, 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 legal.
>
> 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.
>
> I believe this is a negligible corner case and I don't want to add any
> overhead for this; but in debugging kernels, it may be helpful to add
> this check, therefore I put it inside an `#ifdef CONFIG_DEBUG_VM`.

IMHO, we should make is_huge_zero_folio() return the correct answer.
Adding hzf == NULL check should not add much overhead. So removing
#ifdef is better.

>
> 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?"
>
> Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
> ---
>  include/linux/huge_mm.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..e4c617c0b445 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -479,7 +479,12 @@ extern unsigned long huge_zero_pfn;
>
>  static inline bool is_huge_zero_folio(const struct folio *folio)
>  {
> -	return READ_ONCE(huge_zero_folio) == folio;
> +	const struct folio *hzf = READ_ONCE(huge_zero_folio);
> +#ifdef CONFIG_DEBUG_VM
> +	if (hzf == NULL)
> +		return false;
> +#endif
> +	return hzf == folio;
>  }
>
>  static inline bool is_huge_zero_pfn(unsigned long pfn)
> -- 
> 2.47.2


--
Best Regards,
Yan, Zi