Re: [PATCH v6 04/10] KVM: guest_memfd: Allow host to map guest_memfd() pages

Fuad Tabba
2025-03-17T10:43:36Z

Hi Ackerley,

On Fri, 14 Mar 2025 at 18:47, Ackerley Tng <ackerleytng@google.com> wrote:
>
> Fuad Tabba <tabba@google.com> writes:
>
> > 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.
> >
> > Also add the KVM capability KVM_CAP_GMEM_SHARED_MEM, which
> > indicates that the VM supports shared memory in guest_memfd, or
> > that the host can create VMs that support shared memory.
> > Supporting shared memory implies that memory can be mapped when
> > shared with the host.
> >
> > This is controlled by the KVM_GMEM_SHARED_MEM configuration
> > option.
> >
> > Signed-off-by: Fuad Tabba <tabba@google.com>
> > ---
> > include/linux/kvm_host.h | 11 +++++
> > include/uapi/linux/kvm.h | 1 +
> > virt/kvm/guest_memfd.c | 102 +++++++++++++++++++++++++++++++++++++++
> > virt/kvm/kvm_main.c | 4 ++
> > 4 files changed, 118 insertions(+)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 3ad0719bfc4f..601bbcaa5e41 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_GMEM_SHARED_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/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 45e6d8fca9b9..117937a895da 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -929,6 +929,7 @@ struct kvm_enable_cap {
> > #define KVM_CAP_PRE_FAULT_MEMORY 236
> > #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
> > #define KVM_CAP_X86_GUEST_MODE 238
> > +#define KVM_CAP_GMEM_SHARED_MEM 239
> >
> > struct kvm_irq_routing_irqchip {
> > __u32 irqchip;
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index 5fc414becae5..eea44e003ed1 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -320,7 +320,109 @@ 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 bool folio_offset_is_shared(const struct folio *folio, struct file *file, pgoff_t index)
> > +{
> > + struct kvm_gmem *gmem = file->private_data;
> > +
> > + VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>
> I should've commented on this in the last series, but why must folio
> lock be held to check if this offset is shared?
>
> I was thinking to use the filemap's lock (filemap_invalidate_lock()) to
> guard mappability races. Does that work too?

I was thinking the same thing as I am preparing the sharing state
patch series to be sent. I (wrongly) thought before that it wasn't
possible to protect all cases with the invalidate_lock, but they
already are.

I will fix it in the respin of both. Thanks!

/fuad

> > +
> > + /* 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)) {
> > + int err = PTR_ERR(folio);
> > +
> > + if (err == -EAGAIN)
> > + ret = VM_FAULT_RETRY;
> > + else
> > + ret = vmf_error(err);
> > +
> > + goto out_filemap;
> > + }
> > +
> > + if (folio_test_hwpoison(folio)) {
> > + ret = VM_FAULT_HWPOISON;
> > + goto out_folio;
> > + }
> > +
> > + if (!folio_offset_is_shared(folio, vmf->vma->vm_file, vmf->pgoff)) {
> > + ret = VM_FAULT_SIGBUS;
> > + goto out_folio;
> > + }
> > +
> > + /*
> > + * Shared folios would not be marked as "guestmem" so far, and we only
> > + * expect shared 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);
> > + }
> > +
> > + vmf->page = folio_file_page(folio, vmf->pgoff);
> > +
> > +out_folio:
> > + if (ret != VM_FAULT_LOCKED) {
> > + folio_unlock(folio);
> > + folio_put(folio);
> > + }
> > +
> > +out_filemap:
> > + filemap_invalidate_unlock_shared(inode->i_mapping);
> > +
> > + return ret;
> > +}
> > +
> > <snip>