[PATCH RFC v2 33/50] KVM: guest_memfd: Support mmap and user fault handling in PFN mode

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

In PFN mode, guest_memfd memory is backed by raw PFNs rather than
page-cache folios.

Mark the VMA with VM_PFNMAP when setting up the VMA in PFN mode, and
insert the PFN into the user page tables during fault handling.

In folio mode, the fault handler obtains a locked folio under the
shared filemap invalidation lock and returns with the folio still
locked so the core fault handler installs the PTE before unlocking the
folio. Any concurrent truncation or invalidation must wait on the
folio lock before unmapping and removing the folio.

In PFN mode, there is no folio lock to bridge the gap after dropping
the shared filemap invalidation lock. If the lock were dropped before
inserting the PFN into the page tables, a concurrent hole punch or
provider-driven revocation could acquire the exclusive filemap
invalidation lock, unmap existing mappings (missing the not-yet-mapped
PTE), and reclaim the backing memory, only for the fault handler to
subsequently install a stale PTE pointing to the revoked PFN.

Keep the shared filemap invalidation lock held across PFN insertion so
that obtaining the PFN and installing the PTE are atomic with respect
to operations holding the exclusive filemap invalidation lock. For
provider-driven revocation to safely tear down mappings without races,
the provider must first mark the range unavailable so new faults fail,
and then invoke revocation, which acquires the exclusive filemap
invalidation lock to wait for any in-flight faults to finish installing
their PTEs before unmapping them. This also requires providers not to
hold any lock across revocation that is also acquired during PFN
lookup.

Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 virt/kvm/guest_memfd.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 8588912cb0b82..50031e191968b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -562,8 +562,10 @@ static bool kvm_gmem_supports_mmap(struct inode *inode)
 static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 {
 	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct gmem_inode *gi = GMEM_I(inode);
 	vm_fault_t ret = VM_FAULT_LOCKED;
 	struct folio *folio = NULL;
+	kvm_pfn_t pfn;
 	int r = 0;
 
 	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
@@ -572,6 +574,10 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 	filemap_invalidate_lock_shared(inode->i_mapping);
 	if (!kvm_gmem_is_shared_mem(inode, vmf->pgoff)) {
 		r = -EACCES;
+	} else if (kvm_gmem_is_pfn_mode(gi)) {
+		r = gmem_provider_get_pfn(gi, vmf->pgoff, &pfn, NULL);
+		if (!r)
+			ret = vmf_insert_pfn(vmf->vma, vmf->address, pfn);
 	} else {
 		folio = kvm_gmem_get_folio(inode, vmf->pgoff);
 		if (IS_ERR(folio))
@@ -586,6 +592,9 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 		return vmf_error(r);
 	}
 
+	if (kvm_gmem_is_pfn_mode(gi))
+		return ret;
+
 	if (WARN_ON_ONCE(folio_test_large(folio))) {
 		ret = VM_FAULT_SIGBUS;
 		goto out_folio;
@@ -646,6 +655,8 @@ static const struct vm_operations_struct kvm_gmem_vm_ops = {
 
 static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
+	struct gmem_inode *gi = GMEM_I(file_inode(file));
+
 	if (!kvm_gmem_supports_mmap(file_inode(file)))
 		return -ENODEV;
 
@@ -654,6 +665,9 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 	}
 
+	if (kvm_gmem_is_pfn_mode(gi))
+		vm_flags_set(vma, VM_PFNMAP);
+
 	vma->vm_ops = &kvm_gmem_vm_ops;
 
 	return 0;

-- 
2.56.0.257.gf7497c7d27-goog