Re: [RFC PATCH v2 03/11] KVM: guest_memfd: Allow host to map guest_memfd() pages

[Fuad Tabba <tabba@google.com>]

On Fri, 7 Feb 2025 at 16:45, Patrick Roy <roypat@amazon.co.uk> wrote:
>
> Hi Fuad!
>
> On Wed, 2025-01-29 at 17:23 +0000, 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 (shared memory). To that end, this patch adds
> > the ability to check whether the VM type has that support, and
> > only allows mapping its memory if that's the case.
> >
> > Additionally, this behavior is gated with a new configuration
> > option, CONFIG_KVM_GMEM_SHARED_MEM.
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> >
> > ---
> >
> > This patch series will allow shared memory support for software
> > VMs in x86. It will also introduce a similar VM type for arm64
> > and allow shared memory support for that. In the future, pKVM
> > will also support shared memory.
> > ---
> >  include/linux/kvm_host.h | 11 ++++++
> >  virt/kvm/Kconfig         |  4 +++
> >  virt/kvm/guest_memfd.c   | 77 ++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 92 insertions(+)
> >
> > -snip-
> >
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index 47a9f68f7b24..86441581c9ae 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -307,7 +307,84 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
> >         return gfn - slot->base_gfn + slot->gmem.pgoff;
> >  }
> >
> > +#ifdef CONFIG_KVM_GMEM_SHARED_MEM
> > +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)) {
> > +               ret = VM_FAULT_SIGBUS;
> > +               goto out_filemap;
> > +       }
> > +
> > +       if (folio_test_hwpoison(folio)) {
> > +               ret = VM_FAULT_HWPOISON;
> > +               goto out_folio;
> > +       }
> > +
> > +       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_nr_pages(folio) > 1)) {
> > +               ret = VM_FAULT_SIGBUS;
> > +               goto out_folio;
> > +       }
> > +
> > +       if (!folio_test_uptodate(folio)) {
> > +               clear_highpage(folio_page(folio, 0));
> > +               folio_mark_uptodate(folio);
>
> kvm_gmem_mark_prepared() instead of direct folio_mark_uptodate() here, I
> think (in preparation of things like [1])? Noticed this while rebasing
> my direct map removal series on top of this and wondering why mmap'd
> folios sometimes didn't get removed (since it hooks mark_prepared()).

Thanks for pointing that out. Will fix.
/fuad

> Best,
> Patrick
>
> [1]: https://lore.kernel.org/kvm/20241108155056.332412-1-pbonzini@redhat.com/