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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Aug 27, 2025 at 05:03:30PM +0200, 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.

This is true of a huge amount of kernel logic though. We can't always cover
off every single possibility when documenting these things, and there's
often an implicit assumption of this not being null.

>
> 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.

Hmm seems like this is a bug under a bug. folio_put_refs() shouldn't be
passed a folio batch of NULL's.

Shouldn't we just put the VM_WARN_ON_ONCE() there?

>
> 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().

But I really don't think passing NULL to is_huge_zero_folio() is a valid
enough situation to justify this?

You've encountered a case where a bug caused folio_put_refs() to be called
with an invalid parameter, then you're arbitrarily changing
is_huge_zero_folio() so it would deref the folio and splat.

This seems not so great.

I really think the VM_WARN_ON_ONCE() should be in folios_put_refs() based
on what you've said.

>
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  include/linux/huge_mm.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 7748489fde1b..bc9ca7798f2e 100644
> --- 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()

Please don't do //.

This include is suspect though, huge_mm.h is included from mm.h and thus
this very easily might break some arch that is weird about this stuff,
because a ton of stuff includes mm.h including things that might absolutely
baulk at mmdebug.

I've had this kind of thing happen several times before.

>  #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);
> +

Convention is VM_WARN_ON_ONCE(!folio);

>  	return READ_ONCE(huge_zero_folio) == folio;
>  }
>
> --
> 2.47.2
>