[PATCH RFC v2 46/50] samples/kvm: Implement get_pfn provider operation

[Ackerley Tng via B4 Relay <devnull+ackerleytng.google.com@kernel.org>]

From: Ackerley Tng <ackerleytng@google.com>

Implement the .get_pfn callback in mps_gmem_provider_ops to operate as a
direct PFN provider for guest_memfd.

Implementing .get_pfn satisfies guest_memfd's provider operations
requirement, enabling mem_provider_sample files to be attached as
resource_fd memory providers for guest_memfd.

In mps_gmem_get_pfn(), return the physical frame number corresponding to
the requested index from the CMA reservation, returning -EFAULT if the
page was marked absent via MPS_SET_PRESENT.

Checking test_bit(index, info->absent) is done locklessly without
holding info->lock:
- test_bit() is an atomic memory read.
- Locklessness prevents lock inversion and deadlocks. In guest_memfd,
  the guest page fault path (kvm_gmem_get_pfn()) holds
  filemap_invalidate_lock_shared while calling .get_pfn(). Avoiding
  info->lock in .get_pfn() ensures no lock inversions exist between
  filemap_invalidate_lock and info->lock.
- Similarly, while kvm_gmem_revoke() now takes an explicit pfn and avoids
  calling back into .get_pfn(), avoiding info->lock in .get_pfn() keeps
  the lookup path completely lock-free and robust.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 samples/kvm/mem_provider_sample.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/samples/kvm/mem_provider_sample.c b/samples/kvm/mem_provider_sample.c
index d9e57484c3933..a43c76d2259e7 100644
--- a/samples/kvm/mem_provider_sample.c
+++ b/samples/kvm/mem_provider_sample.c
@@ -132,9 +132,29 @@ static void mps_gmem_release(void *provider)
 	fput(resource_file);
 }
 
+static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn,
+			    int *max_order)
+{
+	struct file *resource_file = provider;
+	struct mps_info *info = resource_file->private_data;
+
+	if (index >= info->npages)
+		return -EINVAL;
+
+	if (test_bit(index, info->absent))
+		return -EFAULT;
+
+	*pfn = info->base_pfn + index;
+	if (max_order)
+		*max_order = 0;
+
+	return 0;
+}
+
 static const struct guest_memfd_provider_operations mps_gmem_provider_ops = {
 	.attach		= mps_gmem_attach,
 	.release	= mps_gmem_release,
+	.get_pfn	= mps_gmem_get_pfn,
 };
 
 static const struct file_operations mps_file_fops = {

-- 
2.56.0.257.gf7497c7d27-goog