agent: Verify that we allocated as many hugepages as we need

allocate_hugepages() writes to the kernel sysfs file to allocate hugepages
in the Kata VM.  However, even if the write succeeds, it's not certain that
the kernel will actually be able to allocate as many hugepages as we
requested.

This patch reads back the file after writing it to check if we were able to
allocate all the required hugepages.

fixes #3816

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/agent/src/mount.rs

@@ -351,6 +351,23 @@ fn allocate_hugepages(logger: &Logger, options: &[String]) -> Result<()> {
     file.write_all(numpages.as_bytes())
         .context(format!("write nr_hugepages failed: {:?}", &path))?;
 
+    // Even if the write succeeds, the kernel isn't guaranteed to be
+    // able to allocate all the pages we requested.  Verify that it
+    // did.
+    let verify = fs::read_to_string(&path).context(format!("reading {:?}", &path))?;
+    let allocated = verify
+        .trim_end()
+        .parse::<u64>()
+        .map_err(|_| anyhow!("Unexpected text {:?} in {:?}", &verify, &path))?;
+    if allocated != size / pagesize {
+        return Err(anyhow!(
+            "Only allocated {} of {} hugepages of size {}",
+            allocated,
+            numpages,
+            pagesize
+        ));
+    }
+
     Ok(())
 }