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

[David Hildenbrand <david@redhat.com>]

On 28.08.25 01:53, Andrew Morton wrote:
> 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?

As answered elsewhere, this is perfectly fine as the huge zero folio is 
allocated on demand only (and only once enabled).

> 
> 
> 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.

Common practice to use VM_WARN_ON_ONCE() and friend in inline functions. 
Just look at page-flags.h.

If someone complains about kernel image size with CONFIG_DEBUG_VM, they 
shopuld reevaluate life choices. :)

-- 
Cheers

David / dhildenb