On Sat, Jun 19, 2021 at 02:20:02AM -0700, Peter Collingbourne wrote:

+void prep_refpage_private_data(struct refpage_private_data *priv)
+{
+	u8 *addr = page_address(priv->refpage);
+	u8 pattern = addr[0];
+	int i;
+
+	for (i = 1; i != PAGE_SIZE; ++i)
+		if (addr[i] != pattern)
+			return;
+
+	priv->optzn_kind = REFPAGE_OPTZN_PATTERN;
+	priv->optzn_info = pattern;
+}
+
+void copy_refpage(struct page *page, unsigned long addr,
+		  struct vm_area_struct *vma)
+{
+	struct refpage_private_data *priv = vma->vm_private_data;
+
+	if (priv->optzn_kind == REFPAGE_OPTZN_PATTERN)
+		memset(page_address(page), priv->optzn_info, PAGE_SIZE);
+	else
+		copy_user_highpage(page, priv->refpage, addr, vma);
+}

I wonder if single-byte captures enough of the useful possibilities.
In the kernel we have memset32() and memset64() [1] so we could support
a larger pattern than just an 8-bit byte.  It all depends what userspace
would find useful.

[1] Along with memset_p(), memset_l() and memset16() that aren't terribly
relevant to this use case.  Although maybe memset_l() would be the right
one to use since there probably aren't too many 32-bit apps that want
a 64-bit pattern and memset64() might not be the fastest on a 32-bit
kernel).