linux-kernel - Re: [PATCH 1/2] huge_mm.h: is_huge_zero

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Message-Id: <20250826185515.7fbe1821713195b170ac1b31@linux-foundation.org>
Date: Tue, 26 Aug 2025 18:55:15 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Max Kellermann <max.kellermann@...os.com>
Cc: david@...hat.com, lorenzo.stoakes@...cle.com, ziy@...dia.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 1/2] huge_mm.h: is_huge_zero_folio(NULL) should return
 false

On Wed, 27 Aug 2025 01:16:24 +0200 Max Kellermann <max.kellermann@...os.com> 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.

Isn't it a bug to call is_huge_zero_folio() before the huge_zero_folio
has been created?

Being a simple soul, I'm thinking

	VM_BUG_ON(!huge_zero_folio);
	VM_BUG_ON(!folio);

or similar will settle matters?

> 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`.
> 
> 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?"
> 
> ...
>
> --- 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;
>  }
>  

Yeah, this all seems rather ... complicated.