Re: [PATCH v3 02/11] KVM: guest_memfd: Handle final folio_put() of guest_memfd pages

[Fuad Tabba <tabba@google.com>]

Hi David,

On Thu, 20 Feb 2025 at 11:25, David Hildenbrand <david@redhat.com> wrote:
>
> On 11.02.25 13:11, Fuad Tabba wrote:
> > Before transitioning a guest_memfd folio to unshared, thereby
> > disallowing access by the host and allowing the hypervisor to
> > transition its view of the guest page as private, we need to be
> > sure that the host doesn't have any references to the folio.
> >
> > This patch introduces a new type for guest_memfd folios, which
> > isn't activated in this series but is here as a placeholder and
> > to facilitate the code in the next patch. This will be used in
> > the future to register a callback that informs the guest_memfd
> > subsystem when the last reference is dropped, therefore knowing
> > that the host doesn't have any remaining references.
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
>
> [...]
>
> >   static const char *page_type_name(unsigned int page_type)
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 47bc1bb919cc..241880a46358 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -38,6 +38,10 @@
> >   #include <linux/local_lock.h>
> >   #include <linux/buffer_head.h>
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +#include <linux/kvm_host.h>
> > +#endif
> > +
> >   #include "internal.h"
> >
> >   #define CREATE_TRACE_POINTS
> > @@ -101,6 +105,11 @@ static void free_typed_folio(struct folio *folio)
> >       case PGTY_hugetlb:
> >               free_huge_folio(folio);
> >               return;
> > +#endif
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +     case PGTY_guestmem:
> > +             kvm_gmem_handle_folio_put(folio);
> > +             return;
>
> Hm, if KVM is built as a module, will that work? Or would we need the
> core-mm guest_memfd shim that would always be compiled into the core and
> decouple KVM from guest_memfd ("library")?

I'd assumed that it would work, since the module would have been
loaded before this could trigger, but I guess I was wrong.

Can you point me to an example of a similar shim in the core code, for
me to add on the respin?

Thanks,
/fuad


> --
> Cheers,
>
> David / dhildenb
>