Re: [PATCH hotfix] mm: fix pmd_special() fallback to observe huge_zero

[Lance Yang <lance.yang@linux.dev>]

On 2026/4/30 16:54, David Hildenbrand (Arm) wrote:
>>> ... and triggering the VM_WARN_ON_ONE(), although never reported so far.
>>>
>>> To fix it, teach vm_normal_page_pmd()/vm_normal_page_pud() whether
>>> pmd_special/pud_special is actually implemented.
>>>
>>> Fixes: af38538801c6 ("mm/memory: factor out common code from vm_normal_page_*()")
>>
>> Agreed.  My Fixes tag (where my bisection arrived) was correct for the
>> common zap_huge_pmd() symptom I was seeing (Lorenzo's commit removed
>> an independent is_huge_zero_pmd() check from it, so it now relies on
>> vm_normal_folio_pmd() to give the right answer).  But you've chased
>> up other usages, and realized it goes back further.  You could just as
>> well blame the other commit mentioned in this thread, the d82d09e48219
>> ("mm/huge_memory: mark PMD mappings of the huge zero folio special"),
>> because in come configs it is not doing what it expects to be doing.
>> But af38538801c6 is where effects start appearing, so fine to blame
>> it (and both come from the same 6.18 series, do it doesn't matter).
>>
>>> Signed-off-by: Hugh Dickins <hughd@google.com>>
>>> Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
>>
>> That's generous, but the patch is not mine at all, and I'll
>> happily let you grab my two paragraphs above.  Please, just
> 
> Okay, I didn't want to undermine your involvement.
> 
>> Reported-by: Hugh Dickins <hughd@google.com>
> 
> Then, I'd also add a
> 
> Debugged-by: Hugh Dickins <hughd@google.com>

Yeah. Thanks to Hugh for the bisection, debugging and detailed
analysis!

Cheers, Lance