Re: [PATCH v4 03/10] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Fuad Tabba <tabba@google.com>]

Hi David,

On Thu, 20 Feb 2025 at 12:04, Fuad Tabba <tabba@google.com> wrote:
>
> On Thu, 20 Feb 2025 at 11:58, David Hildenbrand <david@redhat.com> wrote:
> >
> > On 18.02.25 18:24, Fuad Tabba wrote:
> > > Add support for mmap() and fault() for guest_memfd backed memory
> > > in the host for VMs that support in-place conversion between
> > > shared and private. To that end, this patch adds the ability to
> > > check whether the VM type supports in-place conversion, and only
> > > allows mapping its memory if that's the case.
> > >
> > > This behavior is also gated by the configuration option
> > > KVM_GMEM_SHARED_MEM.
> > >
> > > Signed-off-by: Fuad Tabba <tabba@google.com>
> > > ---
> > >   include/linux/kvm_host.h |  11 +++++
> > >   virt/kvm/guest_memfd.c   | 103 +++++++++++++++++++++++++++++++++++++++
> > >   2 files changed, 114 insertions(+)
> > >
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index 3ad0719bfc4f..f9e8b10a4b09 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
> > >   }
> > >   #endif
> > >
> > > +/*
> > > + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
> > > + * private memory is enabled and it supports in-place shared/private conversion.
> > > + */
> > > +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM)
> > > +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
> > > +{
> > > +     return false;
> > > +}
> > > +#endif
> > > +
> > >   #ifndef kvm_arch_has_readonly_mem
> > >   static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
> > >   {
> > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > > index c6f6792bec2a..30b47ff0e6d2 100644
> > > --- a/virt/kvm/guest_memfd.c
> > > +++ b/virt/kvm/guest_memfd.c
> > > @@ -317,9 +317,112 @@ void kvm_gmem_handle_folio_put(struct folio *folio)
> > >   {
> > >       WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
> > >   }
> > > +
> > > +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
> > > +{
> > > +     struct kvm_gmem *gmem = file->private_data;
> > > +
> > > +     /* For now, VMs that support shared memory share all their memory. */
> > > +     return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
> > > +}
> > > +
> > > +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
> > > +{
> > > +     struct inode *inode = file_inode(vmf->vma->vm_file);
> > > +     struct folio *folio;
> > > +     vm_fault_t ret = VM_FAULT_LOCKED;
> > > +
> > > +     filemap_invalidate_lock_shared(inode->i_mapping);
> > > +
> > > +     folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> > > +     if (IS_ERR(folio)) {
> > > +             switch (PTR_ERR(folio)) {
> > > +             case -EAGAIN:
> > > +                     ret = VM_FAULT_RETRY;
> > > +                     break;
> > > +             case -ENOMEM:
> > > +                     ret = VM_FAULT_OOM;
> > > +                     break;
> > > +             default:
> > > +                     ret = VM_FAULT_SIGBUS;
> > > +                     break;
> > > +             }
> > > +             goto out_filemap;
> > > +     }
> > > +
> > > +     if (folio_test_hwpoison(folio)) {
> > > +             ret = VM_FAULT_HWPOISON;
> > > +             goto out_folio;
> > > +     }
> > > +
> > > +     /* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */
> > > +     if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) {
> > > +             ret = VM_FAULT_SIGBUS;
> > > +             goto out_folio;
> > > +     }
> > > +
> > > +     /*
> > > +      * Only private folios are marked as "guestmem" so far, and we never
> > > +      * expect private folios at this point.
> > > +      */
> > > +     if (WARN_ON_ONCE(folio_test_guestmem(folio)))  {
> > > +             ret = VM_FAULT_SIGBUS;
> > > +             goto out_folio;
> > > +     }
> > > +
> > > +     /* No support for huge pages. */
> > > +     if (WARN_ON_ONCE(folio_test_large(folio))) {
> > > +             ret = VM_FAULT_SIGBUS;
> > > +             goto out_folio;
> > > +     }
> > > +
> > > +     if (!folio_test_uptodate(folio)) {
> > > +             clear_highpage(folio_page(folio, 0));
> > > +             kvm_gmem_mark_prepared(folio);
> > > +     }
> >
> > kvm_gmem_get_pfn()->__kvm_gmem_get_pfn() seems to call
> > kvm_gmem_prepare_folio() instead.
> >
> > Could we do the same here?
>
> Will do.

I realized it's not that straightforward. __kvm_gmem_prepare_folio()
requires the kvm_memory_slot, which is used to calculate the gfn. At
that point we have neither, and it's not just an issue of access, but
there might not be a slot associated with that yet.

Cheers,
/fuad
>
> Thanks,
> /fuad
>
> > --
> > Cheers,
> >
> > David / dhildenb
> >