Add flags to set the io_uring mlock rlimit

Summary: Add flags to set the io_uring mlock rlimit

Reviewed By: danobi, kevin-vigor

Differential Revision: D24063687

fbshipit-source-id: be7cdad10797c087a21370ce42811a9752e4f61a

folly/experimental/io/IoUringBackend.cpp

@@ -24,6 +24,12 @@
 
 #include <glog/logging.h>
 
+static constexpr int64_t kUnlimitedMlock = -1;
+DEFINE_int64(
+    io_uring_mlock_size,
+    kUnlimitedMlock,
+    "Maximum bytes to mlock - use 0 for no changes, -1 for unlimited");
+
 namespace {
 class SQGroupInfoRegistry {
  private:
@@ -248,6 +254,23 @@ bool IoUringBackend::FdRegistry::free(
 IoUringBackend::IoUringBackend(Options options)
     : PollIoBackend(options),
       fdRegistry_(ioRing_, options.useRegisteredFds ? options.capacity : 0) {
+  FOLLY_MAYBE_UNUSED static bool sMlockInit = []() {
+    int ret = 0;
+    if (FLAGS_io_uring_mlock_size) {
+      struct rlimit rlim;
+      if (FLAGS_io_uring_mlock_size == kUnlimitedMlock) {
+        rlim.rlim_cur = RLIM_INFINITY;
+        rlim.rlim_max = RLIM_INFINITY;
+      } else {
+        rlim.rlim_cur = FLAGS_io_uring_mlock_size;
+        rlim.rlim_max = FLAGS_io_uring_mlock_size;
+      }
+      ret = setrlimit(RLIMIT_MEMLOCK, &rlim); // best effort
+    }
+
+    return ret ? false : true;
+  }();
+
   ::memset(&ioRing_, 0, sizeof(ioRing_));
   ::memset(&params_, 0, sizeof(params_));