test(uffd): MAP_POPULATE backing file

We use example uffd handlers in performance tests to measure the
population latency.  We do not want for it to be diluted by the page
faults in the backing file.  Using an already populated backing file
makes the measurement more precise.

Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

src/firecracker/examples/uffd/uffd_utils.rs

@@ -225,7 +225,7 @@ impl Runtime {
                 ptr::null_mut(),
                 backing_memory_size,
                 libc::PROT_READ,
-                libc::MAP_PRIVATE,
+                libc::MAP_PRIVATE | libc::MAP_POPULATE,
                 backing_file.as_raw_fd(),
                 0,
             )