Re: [PATCH v2] huge_mm.h: disallow is_huge_zero_folio(NULL)

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 27 Aug 2025 17:03:30 +0200 Max Kellermann <max.kellermann@ionos.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 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().
>
> ...
>
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -2,6 +2,7 @@
>  #ifndef _LINUX_HUGE_MM_H
>  #define _LINUX_HUGE_MM_H
>  
> +#include <linux/mmdebug.h> // for VM_WARN_ON_ONCE()
>  #include <linux/mm_types.h>
>  
>  #include <linux/fs.h> /* only for vma_is_dax() */
> @@ -479,6 +480,8 @@ extern unsigned long huge_zero_pfn;
>  
>  static inline bool is_huge_zero_folio(const struct folio *folio)
>  {
> +	VM_WARN_ON_ONCE(folio == NULL);
> +
>  	return READ_ONCE(huge_zero_folio) == folio;
>  }

OK, but it remains the case that we have seen code which calls
is_huge_zero_folio() prior to the initialization of huge_zero_folio.

Is this a bug?  I think so.  Should we be checking for recurrences of
this bug?


Also, sigh.  I do dislike seeing VM_WARN_ON_ONCE() in an inline
function - heaven knows how much bloat that adds.  Defconfig
mm/huge_memory.o (which has three calls) grows by 80 bytes so I guess
that's livable with.