Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct map

[Nikita Kalyazin]

On 16/01/2026 00:00, Edgecombe, Rick P wrote:
> On Wed, 2026-01-14 at 13:46 +0000, Kalyazin, Nikita wrote:
> > +static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
> > +{
> > +     /*
> > +      * Direct map restoration cannot fail, as the only error condition
> > +      * for direct map manipulation is failure to allocate page tables
> > +      * when splitting huge pages, but this split would have already
> > +      * happened in folio_zap_direct_map() in kvm_gmem_folio_zap_direct_map().
> > +      * Thus folio_restore_direct_map() here only updates prot bits.
> > +      */
> > +     if (kvm_gmem_folio_no_direct_map(folio)) {
> > +             WARN_ON_ONCE(folio_restore_direct_map(folio));
> > +             folio->private = (void *)((u64)folio->private & ~KVM_GMEM_FOLIO_NO_DIRECT_MAP);
> > +     }
> > +}
> > +
>
> Does this assume the folio would not have been split after it was zapped? As in,
> if it was zapped at 2MB granularity (no 4KB direct map split required) but then
> restored at 4KB (split required)? Or it gets merged somehow before this?

AFAIK it can't be zapped at 2MB granularity as the zapping code will 
inevitably cause splitting because guest_memfd faults occur at the base 
page granularity as of now.