From 8a9847b189b05e3dcff81adc257c10050dd777e7 Mon Sep 17 00:00:00 2001
From: Dean Michael Berris <dberris@google.com>
Date: Mon, 30 Jul 2018 05:56:42 +0000
Subject: [XRay][compiler-rt] FDR Mode: Use mmap instead of internal allocator

Summary:
This change moves FDR mode to use `internal_mmap(...)` from
sanitizer_common instead of the internal allocator interface. We're
doing this to sidestep the alignment issues we encounter with the
`InternalAlloc(...)` functions returning pointers that have some magic
bytes at the beginning.

XRay copies bytes into the buffer memory, and does not require the magic
bytes tracking the other sanitizers use when allocating/deallocating
buffers.

Reviewers: kpw, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49972

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@338228 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/xray/xray_buffer_queue.cc | 55 ++++++++++++++++++++++++++++++++++---------
 1 file changed, 44 insertions(+), 11 deletions(-)

diff --git a/lib/xray/xray_buffer_queue.cc b/lib/xray/xray_buffer_queue.cc
index 8dfcc2354..3ce728900 100644
--- a/lib/xray/xray_buffer_queue.cc
+++ b/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 @@ BufferQueue::BufferQueue(size_t B, size_t N, bool &Success)
     // 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 @@ BufferQueue::BufferQueue(size_t B, size_t N, bool &Success)
     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 @@ BufferQueue::~BufferQueue() {
   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);
 }
-- 
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