[PATCH RFC v2 47/50] samples/kvm: Implement put_pfn_range provider operation

[Ackerley Tng via B4 Relay]

From: Ackerley Tng <ackerleytng@xxxxxxxxxx>

When guest_memfd hole punches or destroys an instance backed by a PFN
provider, it invokes .put_pfn_range to release the physical memory back
to the provider.

Implement the .put_pfn_range callback in mps_gmem_provider_ops to clear
the CMA pages across the released range via clear_highpage(). This
ensures that subsequent accesses to punched ranges observe zeroed memory,
matching POSIX fallocate hole-punching semantics and preventing stale data
from being observed when pages are reused.

Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 samples/kvm/mem_provider_sample.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/samples/kvm/mem_provider_sample.c b/samples/kvm/mem_provider_sample.c
index a43c76d2259e7..dee15179959b2 100644
--- a/samples/kvm/mem_provider_sample.c
+++ b/samples/kvm/mem_provider_sample.c
@@ -17,6 +17,7 @@
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/gfp.h>
+#include <linux/highmem.h>
 #include <linux/guest_memfd.h>
 #include <linux/io.h>
 #include <linux/miscdevice.h>
@@ -40,6 +41,21 @@ struct mps_info {
 	unsigned long *absent;		/* taken away by MPS_SET_PRESENT */
 };
 
+static void mps_gmem_put_pfn_range(void *provider, pgoff_t index,
+				   size_t nr_pages)
+{
+	struct file *resource_file = provider;
+	struct mps_info *info = resource_file->private_data;
+	size_t i;
+
+	if (index >= info->npages)
+		return;
+
+	nr_pages = min_t(size_t, nr_pages, info->npages - index);
+	for (i = 0; i < nr_pages; i++)
+		clear_highpage(info->cma_pages + index + i);
+}
+
 /* Byte range [offset, offset + len) of @info as page indices. */
 static int mps_range(struct mps_info *info, u64 offset, u64 len,
 		     unsigned long *start, unsigned long *end)
@@ -155,6 +171,7 @@ 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,
+	.put_pfn_range	= mps_gmem_put_pfn_range,
 };
 
 static const struct file_operations mps_file_fops = {

-- 
2.56.0.257.gf7497c7d27-goog