diff --git a/libc/bionic/pthread_create.cpp b/libc/bionic/pthread_create.cpp
index 9b45161..7e74dac 100644
--- a/libc/bionic/pthread_create.cpp
+++ b/libc/bionic/pthread_create.cpp
@@ -52,8 +52,9 @@
 
 // This code is used both by each new pthread and the code that initializes the main thread.
 void __init_tls(pthread_internal_t* thread) {
-  if (thread->user_allocated_stack()) {
-    // We don't know where the user got their stack, so assume the worst and zero the TLS area.
+  if (thread->mmap_size == 0) {
+    // If the TLS area was not allocated by mmap(), it may not have been cleared to zero.
+    // So assume the worst and zero the TLS area.
     memset(&thread->tls[0], 0, BIONIC_TLS_SLOTS * sizeof(void*));
   }
 
@@ -106,62 +107,62 @@
   return error;
 }
 
-static void* __create_thread_stack(size_t stack_size, size_t guard_size) {
+static void* __create_thread_mapped_space(size_t mmap_size, size_t stack_guard_size) {
   // Create a new private anonymous map.
   int prot = PROT_READ | PROT_WRITE;
   int flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE;
-  void* stack = mmap(NULL, stack_size, prot, flags, -1, 0);
-  if (stack == MAP_FAILED) {
+  void* space = mmap(NULL, mmap_size, prot, flags, -1, 0);
+  if (space == MAP_FAILED) {
     __libc_format_log(ANDROID_LOG_WARN,
                       "libc",
-                      "pthread_create failed: couldn't allocate %zd-byte stack: %s",
-                      stack_size, strerror(errno));
+                      "pthread_create failed: couldn't allocate %zu-bytes mapped space: %s",
+                      mmap_size, strerror(errno));
     return NULL;
   }
 
-  // Set the guard region at the end of the stack to PROT_NONE.
-  if (mprotect(stack, guard_size, PROT_NONE) == -1) {
+  // Stack is at the lower end of mapped space, stack guard region is at the lower end of stack.
+  // Set the stack guard region to PROT_NONE, so we can detect thread stack overflow.
+  if (mprotect(space, stack_guard_size, PROT_NONE) == -1) {
     __libc_format_log(ANDROID_LOG_WARN, "libc",
-                      "pthread_create failed: couldn't mprotect PROT_NONE %zd-byte stack guard region: %s",
-                      guard_size, strerror(errno));
-    munmap(stack, stack_size);
+                      "pthread_create failed: couldn't mprotect PROT_NONE %zu-byte stack guard region: %s",
+                      stack_guard_size, strerror(errno));
+    munmap(space, mmap_size);
     return NULL;
   }
 
-  return stack;
+  return space;
 }
 
 static int __allocate_thread(pthread_attr_t* attr, pthread_internal_t** threadp, void** child_stack) {
-  size_t allocate_stack_size;
+  size_t mmap_size;
   uint8_t* stack_top;
 
   if (attr->stack_base == NULL) {
     // The caller didn't provide a stack, so allocate one.
     // Make sure the stack size and guard size are multiples of PAGE_SIZE.
-    allocate_stack_size = BIONIC_ALIGN(attr->stack_size + sizeof(pthread_internal_t), PAGE_SIZE);
+    mmap_size = BIONIC_ALIGN(attr->stack_size + sizeof(pthread_internal_t), PAGE_SIZE);
     attr->guard_size = BIONIC_ALIGN(attr->guard_size, PAGE_SIZE);
-    attr->stack_base = __create_thread_stack(allocate_stack_size, attr->guard_size);
+    attr->stack_base = __create_thread_mapped_space(mmap_size, attr->guard_size);
     if (attr->stack_base == NULL) {
       return EAGAIN;
     }
-    stack_top = reinterpret_cast<uint8_t*>(attr->stack_base) + allocate_stack_size;
+    stack_top = reinterpret_cast<uint8_t*>(attr->stack_base) + mmap_size;
   } else {
-    // The caller did provide a stack, so remember we're not supposed to free it.
-    attr->flags |= PTHREAD_ATTR_FLAG_USER_ALLOCATED_STACK;
-    allocate_stack_size = 0;
+    // Remember the mmap size is zero and we don't need to free it.
+    mmap_size = 0;
     stack_top = reinterpret_cast<uint8_t*>(attr->stack_base) + attr->stack_size;
   }
 
-  // Thread stack is used for two sections:
-  //   pthread_internal_t.
-  //   regular stack, from top to down.
+  // Mapped space(or user allocated stack) is used for:
+  //   thread_internal_t (including tls array)
+  //   thread stack (including guard page)
   stack_top -= sizeof(pthread_internal_t);
   pthread_internal_t* thread = reinterpret_cast<pthread_internal_t*>(stack_top);
 
   // No need to check stack_top alignment. The size of pthread_internal_t is 16-bytes aligned,
   // and user allocated stack is guaranteed by pthread_attr_setstack.
 
-  thread->allocated_stack_size = allocate_stack_size;
+  thread->mmap_size = mmap_size;
   thread->attr = *attr;
   __init_tls(thread);
 
@@ -248,8 +249,8 @@
     // be unblocked, but we're about to unmap the memory the mutex is stored in, so this serves as a
     // reminder that you can't rewrite this function to use a ScopedPthreadMutexLocker.
     pthread_mutex_unlock(&thread->startup_handshake_mutex);
-    if (!thread->user_allocated_stack()) {
-      munmap(thread->attr.stack_base, thread->allocated_stack_size);
+    if (thread->mmap_size != 0) {
+      munmap(thread->attr.stack_base, thread->mmap_size);
     }
     __libc_format_log(ANDROID_LOG_WARN, "libc", "pthread_create failed: clone failed: %s", strerror(errno));
     return clone_errno;