[RFC PATCH 13/14] samples/kvm: Back absent pages on demand in the sample provider

[Fred Griffoul <griffoul@gmail.com>]

From: Fred Griffoul <fgriffo@amazon.co.uk>

The sample provider fails get_pfn() for an absent page and keeps its
frames on a punch hole, so tests cannot exercise a provider that backs
a hole when it is asked for a frame, one that backs more than the page
asked for, or one that frees the frames that a punch hole gives back.

Add MPS_NEW_FILE_FILL: get_pfn() then backs an absent page with the
whole 2 MiB block around it, and put_pfn_range() makes the pages absent
again.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Fred Griffoul <fgriffo@amazon.co.uk>
---
 samples/kvm/mem_provider_sample.c | 29 ++++++++++++++++++++++++-----
 samples/kvm/mem_provider_sample.h |  9 ++++++++-
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/samples/kvm/mem_provider_sample.c b/samples/kvm/mem_provider_sample.c
index 670e36207e59..41e0ce5b6bc6 100644
--- a/samples/kvm/mem_provider_sample.c
+++ b/samples/kvm/mem_provider_sample.c
@@ -41,6 +41,7 @@ struct mps_info {
 
 	unsigned long *absent;		/* taken away by MPS_SET_PRESENT */
 	unsigned long *readonly;	/* set by MPS_SET_READONLY */
+	bool fill;			/* MPS_NEW_FILE_FILL */
 
 	struct inode *gmem_inode;
 };
@@ -62,8 +63,15 @@ 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;
 
-	mps_zero_range(resource_file->private_data, index, nr_pages);
+	mps_zero_range(info, index, nr_pages);
+	if (!info->fill || index >= info->npages)
+		return;
+	/* Lockless, like get_pfn(): guest_memfd holds its invalidate lock. */
+	for (i = index; i < min_t(size_t, index + nr_pages, info->npages); i++)
+		set_bit(i, info->absent);
 }
 
 /* Byte range [offset, offset + len) of @info as page indices. */
@@ -228,7 +236,9 @@ static bool mps_bits_are(unsigned long *bitmap, unsigned long first,
 /*
  * The frame behind @index, and the largest aligned block around it that is
  * present and has the same attributes: the file's frames are contiguous.
- * Lockless: a change is followed by kvm_gmem_revoke() or kvm_gmem_restore().
+ * Lockless: a change is followed by kvm_gmem_revoke() or kvm_gmem_restore(),
+ * except a fill, which only gives frames to holes; guest_memfd tells its users
+ * about the block.
  */
 static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn,
 			    int *max_order, u32 *attrs)
@@ -241,8 +251,16 @@ static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn,
 	if (index >= info->npages)
 		return -EINVAL;
 
-	if (test_bit(index, info->absent))
-		return -EFAULT;
+	if (test_bit(index, info->absent)) {
+		unsigned long first = ALIGN_DOWN(index, PTRS_PER_PMD);
+		unsigned long i;
+
+		if (!info->fill)
+			return -EFAULT;
+		/* Absent pages are zeroed already. */
+		for (i = first; i < min(first + PTRS_PER_PMD, info->npages); i++)
+			clear_bit(i, info->absent);
+	}
 
 	ro = test_bit(index, info->readonly);
 	*pfn = info->base_pfn + index;
@@ -307,7 +325,7 @@ static long mps_ctl_new_file(void __user *uarg)
 
 	if (copy_from_user(&nf, uarg, sizeof(nf)))
 		return -EFAULT;
-	if (nf.flags || nf.pad)
+	if ((nf.flags & ~MPS_NEW_FILE_FILL) || nf.pad)
 		return -EINVAL;
 	if (!nf.size || !PAGE_ALIGNED(nf.size))
 		return -EINVAL;
@@ -337,6 +355,7 @@ static long mps_ctl_new_file(void __user *uarg)
 	info->cma_pages = pages;
 	info->base_pfn = page_to_pfn(pages);
 	info->npages = npages;
+	info->fill = nf.flags & MPS_NEW_FILE_FILL;
 	mutex_init(&info->lock);
 
 	fd = get_unused_fd_flags(O_CLOEXEC);
diff --git a/samples/kvm/mem_provider_sample.h b/samples/kvm/mem_provider_sample.h
index 57f494dc46fa..45ec9605b593 100644
--- a/samples/kvm/mem_provider_sample.h
+++ b/samples/kvm/mem_provider_sample.h
@@ -19,11 +19,18 @@
  * Returns the provider fd.
  */
 struct mps_new_file {
-	__u32 flags;	/* Reserved, must be 0 */
+	__u32 flags;	/* MPS_NEW_FILE_* */
 	__u32 pad;
 	__u64 size;	/* bytes, page aligned */
 };
 
+/*
+ * get_pfn() backs an absent page, with every page of the 2 MiB block around
+ * it, instead of failing, and put_pfn_range() takes the pages back: a
+ * provider that allocates on demand and frees on a punch hole, for tests.
+ */
+#define MPS_NEW_FILE_FILL	(1U << 0)
+
 #define MPS_NEW_FILE \
 	_IOW(MPS_IOCTL_BASE, 5, struct mps_new_file)
 
-- 
2.47.3