[PATCH RFC v4 07/44] KVM: guest_memfd: Only prepare folios for private pages

[Ackerley Tng]

All-shared guest_memfd used to be only supported for non-CoCo VMs where
preparation doesn't apply. INIT_SHARED is about to be supported for
non-CoCo VMs in a later patch in this series.

In addition, KVM_SET_MEMORY_ATTRIBUTES2 is about to be supported in
guest_memfd in a later patch in this series.

This means that the kvm fault handler may now call kvm_gmem_get_pfn() on a
shared folio for a CoCo VM where preparation applies.

Add a check to make sure that preparation is only performed for private
folios.

Preparation will be undone on freeing (see kvm_gmem_free_folio()) and on
conversion to shared.

Signed-off-by: Ackerley Tng <ackerley...@google.com>
---
 virt/kvm/guest_memfd.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index b6ffa8734175d..d414ebfcb4c19 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -900,6 +900,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
kvm_memory_slot *slot,
                     int *max_order)
 {
        pgoff_t index = kvm_gmem_get_index(slot, gfn);
+       struct inode *inode;
        struct folio *folio;
        int r = 0;
 
@@ -907,7 +908,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
kvm_memory_slot *slot,
        if (!file)
                return -EFAULT;
 
-       filemap_invalidate_lock_shared(file_inode(file)->i_mapping);
+       inode = file_inode(file);
+       filemap_invalidate_lock_shared(inode->i_mapping);
 
        folio = __kvm_gmem_get_pfn(file, slot, index, pfn, max_order);
        if (IS_ERR(folio)) {
@@ -920,7 +922,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
kvm_memory_slot *slot,
                folio_mark_uptodate(folio);
        }
 
-       r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
+       if (kvm_gmem_is_private_mem(inode, index))
+               r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
 
        folio_unlock(folio);
 
@@ -930,7 +933,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct 
kvm_memory_slot *slot,
                folio_put(folio);
 
 out:
-       filemap_invalidate_unlock_shared(file_inode(file)->i_mapping);
+       filemap_invalidate_unlock_shared(inode->i_mapping);
        return r;
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);

-- 
2.53.0.1018.g2bb0e51243-goog