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

[David Hildenbrand <david@xxxxxxxxxx>]

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