Re: [PATCH v8 05/13] KVM: guest_memfd: Add flag to remove from direct map

[Nikita Kalyazin]

On 05/12/2025 17:30, Dave Hansen wrote:
> On 12/5/25 08:58, 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 set_direct_map_invalid_noflush() in kvm_gmem_folio_zap_direct_map().
> > +      * Thus set_direct_map_valid_noflush() here only updates prot bits.
> > +      */
> > +     if (kvm_gmem_folio_no_direct_map(folio))
> > +             set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
> > +                                      true);
> > +}
>
> This is rather hopeful programming.
>
> I can think of a number of ways that this assumption might become invalid.
> This at *least* check for set_direct_map_valid_noflush() failures (or
> whatever interface you end up using)?
>
> A WARN_ON_ONCE() would be fine.

Done in v9, thanks.