On Fri, Feb 26, 2021 at 06:18:29PM +1100, Alistair Popple wrote:

+/**
+ * make_device_exclusive_range() - Mark a range for exclusive use by a device
+ * @mm: mm_struct of assoicated target process
+ * @start: start of the region to mark for exclusive device access
+ * @end: end address of region
+ * @pages: returns the pages which were successfully mark for exclusive acces
+ *
+ * Returns: number of pages successfully marked for exclusive access
+ *
+ * This function finds the ptes mapping page(s) to the given address range and
+ * replaces them with special swap entries preventing userspace CPU access. On
+ * fault these entries are replaced with the original mapping after calling MMU
+ * notifiers.
+ */
+int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
+				unsigned long end, struct page **pages)
+{
+	long npages = (end - start) >> PAGE_SHIFT;
+	long i;
+
+	npages = get_user_pages_remote(mm, start, npages,
+				       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
+				       pages, NULL, NULL);
+	for (i = 0; i < npages; i++) {
+		if (!trylock_page(pages[i])) {
+			put_page(pages[i]);
+			pages[i] = NULL;
+			continue;
+		}
+
+		if (!try_to_protect(pages[i])) {

Isn't this racy? get_user_pages returns the ptes at an instant in
time, they could have already been changed to something else?

I would think you'd want to switch to the swap entry atomically under
th PTLs?

Jason