Index: compiler-rt/lib/xray/xray_buffer_queue.cc
===================================================================
--- compiler-rt/lib/xray/xray_buffer_queue.cc
+++ compiler-rt/lib/xray/xray_buffer_queue.cc
@@ -13,17 +13,50 @@
 //
 //===----------------------------------------------------------------------===//
 #include "xray_buffer_queue.h"
-#include "sanitizer_common/sanitizer_allocator_internal.h"
 #include "sanitizer_common/sanitizer_common.h"
 #include "sanitizer_common/sanitizer_libc.h"
+#include "sanitizer_common/sanitizer_posix.h"
 #include <memory>
+#include <sys/mman.h>
+
+#ifndef MAP_NORESERVE
+// no-op on NetBSD (at least), unsupported flag on FreeBSD
+#define MAP_NORESERVE 0
+#endif
 
 using namespace __xray;
 using namespace __sanitizer;
 
+template <class T> static T *allocRaw(size_t N) {
+  // TODO: Report errors?
+  // We use MAP_NORESERVE on platforms where it's supported to ensure that the
+  // pages we're allocating for XRay never end up in pages that can be swapped
+  // in/out. We're doing this because for FDR mode, we want to ensure that
+  // writes to the buffers stay resident in memory to prevent XRay itself from
+  // causing swapping/thrashing.
+  //
+  // In the case when XRay pages cannot be swapped in/out or there's not enough
+  // RAM to back these pages, we're willing to cause a segmentation fault
+  // instead of introducing latency in the measurement. We assume here that
+  // there are enough pages that are swappable in/out outside of the buffers
+  // being used by FDR mode (which are bounded and configurable anyway) to allow
+  // us to keep using always-resident memory.
+  //
+  // TODO: Make this configurable?
+  void *A = reinterpret_cast<void *>(
+      internal_mmap(NULL, N * sizeof(T), PROT_WRITE | PROT_READ,
+                    MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE, -1, 0));
+  return (A == MAP_FAILED) ? nullptr : reinterpret_cast<T *>(A);
+}
+
+template <class T> static void deallocRaw(T *ptr, size_t N) {
+  // TODO: Report errors?
+  if (ptr != nullptr)
+    internal_munmap(ptr, N);
+}
+
 template <class T> static T *initArray(size_t N) {
-  auto A = reinterpret_cast<T *>(
-      InternalAlloc(N * sizeof(T), nullptr, kCacheLineSize));
+  auto A = allocRaw<T>(N);
   if (A != nullptr)
     while (N > 0)
       new (A + (--N)) T();
@@ -42,19 +75,19 @@
     // Clean up the buffers we've already allocated.
     for (auto B = Buffers, E = Buffers + BufferCount; B != E; ++B)
       B->~BufferRep();
-    InternalFree(Buffers);
+    deallocRaw(Buffers, N);
     Success = false;
     return;
   };
 
   for (size_t i = 0; i < N; ++i) {
     auto &T = Buffers[i];
-    void *Tmp = InternalAlloc(BufferSize, nullptr, 64);
+    void *Tmp = allocRaw<char>(BufferSize);
     if (Tmp == nullptr) {
       Success = false;
       return;
     }
-    void *Extents = InternalAlloc(sizeof(BufferExtents), nullptr, 64);
+    auto *Extents = allocRaw<BufferExtents>(1);
     if (Extents == nullptr) {
       Success = false;
       return;
@@ -62,7 +95,7 @@
     auto &Buf = T.Buff;
     Buf.Data = Tmp;
     Buf.Size = B;
-    Buf.Extents = reinterpret_cast<BufferExtents *>(Extents);
+    Buf.Extents = Extents;
     OwnedBuffers[i] = Tmp;
   }
   Success = true;
@@ -128,11 +161,11 @@
   for (auto I = Buffers, E = Buffers + BufferCount; I != E; ++I) {
     auto &T = *I;
     auto &Buf = T.Buff;
-    InternalFree(Buf.Data);
-    InternalFree(Buf.Extents);
+    deallocRaw(Buf.Data, Buf.Size);
+    deallocRaw(Buf.Extents, 1);
   }
   for (auto B = Buffers, E = Buffers + BufferCount; B != E; ++B)
     B->~BufferRep();
-  InternalFree(Buffers);
-  InternalFree(OwnedBuffers);
+  deallocRaw(Buffers, BufferCount);
+  deallocRaw(OwnedBuffers, BufferCount);
 }