[XRay][compiler-rt] Remove MAP_NORESERVE from XRay allocations

[dberris]

Summary

This reverses an earlier decision to allow seg-faulting from the
XRay-allocated memory if it turns out that the system cannot provide
physical memory backing that cannot be swapped in/out on Linux.

This addresses http://llvm.org/PR38588.

compiler-rt/lib/xray/xray_allocator.h

 #ifndef XRAY_ALLOCATOR_H
 #define XRAY_ALLOCATOR_H
 
 #include "sanitizer_common/sanitizer_common.h"
 #include "sanitizer_common/sanitizer_internal_defs.h"
 #include "sanitizer_common/sanitizer_mutex.h"
 #include "sanitizer_common/sanitizer_posix.h"
 #include "xray_utils.h"
-#include <sys/mman.h>
 #include <cstddef>
 #include <cstdint>
-
-#ifndef MAP_NORESERVE
-// no-op on NetBSD (at least), unsupported flag on FreeBSD basically because unneeded
-#define MAP_NORESERVE 0
-#endif
+#include <sys/mman.h>
 
 namespace __xray {
 
 /// The Allocator type hands out fixed-sized chunks of memory that are
 /// cache-line aligned and sized. This is useful for placement of
 /// performance-sensitive data in memory that's frequently accessed. The
 /// allocator also self-limits the peak memory usage to a dynamically defined
 /// maximum.
private:
   size_t AllocatedBlocks = 0;
   SpinMutex Mutex{};
 
   void *Alloc() {
     SpinMutexLock Lock(&Mutex);
     if (UNLIKELY(BackingStore == nullptr)) {
       BackingStore = reinterpret_cast<void *>(
           internal_mmap(NULL, MaxMemory, PROT_READ | PROT_WRITE,
-                        MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE, 0, 0));
+                        MAP_PRIVATE | MAP_ANONYMOUS, 0, 0));
       if (BackingStore == MAP_FAILED) {
         BackingStore = nullptr;
         if (Verbosity())
           Report("XRay Profiling: Failed to allocate memory for allocator.\n");
         return nullptr;
       }
 
       AlignedNextBlock = BackingStore;

compiler-rt/lib/xray/xray_buffer_queue.cc

 //===----------------------------------------------------------------------===//
 #include "xray_buffer_queue.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));
+                    MAP_ANONYMOUS | MAP_PRIVATE, -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);
 }