Re: [PATCH v1 4/9] fs/dax: use vmf_insert_folio_pmd() to insert the huge zero folio

[David Hildenbrand <david@redhat.com>]

On 17.07.25 10:38, Alistair Popple wrote:
> On Tue, Jul 15, 2025 at 03:23:45PM +0200, David Hildenbrand wrote:
>> Let's convert to vmf_insert_folio_pmd().
>>
>> There is a theoretical change in behavior: in the unlikely case there is
>> already something mapped, we'll now still call trace_dax_pmd_load_hole()
>> and return VM_FAULT_NOPAGE.
>>
>> Previously, we would have returned VM_FAULT_FALLBACK, and the caller
>> would have zapped the PMD to try a PTE fault.
>>
>> However, that behavior was different to other PTE+PMD faults, when there
>> would already be something mapped, and it's not even clear if it could
>> be triggered.
>>
>> Assuming the huge zero folio is already mapped, all good, no need to
>> fallback to PTEs.
>>
>> Assuming there is already a leaf page table ... the behavior would be
>> just like when trying to insert a PMD mapping a folio through
>> dax_fault_iter()->vmf_insert_folio_pmd().
>>
>> Assuming there is already something else mapped as PMD? It sounds like
>> a BUG, and the behavior would be just like when trying to insert a PMD
>> mapping a folio through dax_fault_iter()->vmf_insert_folio_pmd().
>>
>> So, it sounds reasonable to not handle huge zero folios differently
>> to inserting PMDs mapping folios when there already is something mapped.
> 
> Yeah, this all sounds reasonable and I was never able to hit this path with the
> RFC version of this series anyway. So I suspect it really is impossible to hit
> and therefore any change is theoretical.

Thanks for the review and test, Alistair!

-- 
Cheers,

David / dhildenb