diff --git a/libc/malloc_debug/MapData.cpp b/libc/malloc_debug/MapData.cpp
index b22c109..c58882a 100644
--- a/libc/malloc_debug/MapData.cpp
+++ b/libc/malloc_debug/MapData.cpp
@@ -34,6 +34,8 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <sys/uio.h>
+#include <unistd.h>
 
 #include <vector>
 
@@ -69,148 +71,132 @@
 
   MapEntry* entry = new MapEntry(start, end, offset, name, name_len, flags);
   if (!(flags & PROT_READ)) {
-    // Any unreadable map will just get a zero load bias.
-    entry->load_bias = 0;
-    entry->init = true;
-    entry->valid = false;
+    // This will make sure that an unreadable map will prevent attempts to read
+    // elf data from the map.
+    entry->SetInvalid();
   }
   return entry;
 }
 
-template <typename T>
-static inline bool get_val(MapEntry* entry, uintptr_t addr, T* store) {
-  if (!(entry->flags & PROT_READ) || addr < entry->start || addr + sizeof(T) > entry->end) {
-    return false;
+void MapEntry::Init() {
+  if (init_) {
+    return;
   }
-  // Make sure the address is aligned properly.
-  if (addr & (sizeof(T) - 1)) {
-    return false;
-  }
-  *store = *reinterpret_cast<T*>(addr);
-  return true;
-}
+  init_ = true;
 
-static bool valid_elf(MapEntry* entry) {
-  uintptr_t addr = entry->start;
-  uintptr_t end;
-  if (__builtin_add_overflow(addr, SELFMAG, &end) || end >= entry->end) {
-    return false;
+  uintptr_t end_addr;
+  if (__builtin_add_overflow(start_, SELFMAG, &end_addr) || end_addr >= end_) {
+    return;
   }
 
-  return memcmp(reinterpret_cast<void*>(addr), ELFMAG, SELFMAG) == 0;
-}
-
-static void read_loadbias(MapEntry* entry) {
-  entry->load_bias = 0;
-  uintptr_t addr = entry->start;
   ElfW(Ehdr) ehdr;
-  if (!get_val<ElfW(Half)>(entry, addr + offsetof(ElfW(Ehdr), e_phnum), &ehdr.e_phnum)) {
-    return;
+  struct iovec src_io = {.iov_base = reinterpret_cast<void*>(start_), .iov_len = SELFMAG};
+  struct iovec dst_io = {.iov_base = ehdr.e_ident, .iov_len = SELFMAG};
+  ssize_t rc = process_vm_readv(getpid(), &dst_io, 1, &src_io, 1, 0);
+  valid_ = rc == SELFMAG && IS_ELF(ehdr);
+}
+
+uintptr_t MapEntry::GetLoadBias() {
+  if (!valid_) {
+    return 0;
   }
-  if (!get_val<ElfW(Off)>(entry, addr + offsetof(ElfW(Ehdr), e_phoff), &ehdr.e_phoff)) {
-    return;
+
+  if (load_bias_read_) {
+    return load_bias_;
   }
-  addr += ehdr.e_phoff;
+
+  load_bias_read_ = true;
+
+  ElfW(Ehdr) ehdr;
+  struct iovec src_io = {.iov_base = reinterpret_cast<void*>(start_), .iov_len = sizeof(ehdr)};
+  struct iovec dst_io = {.iov_base = &ehdr, .iov_len = sizeof(ehdr)};
+  ssize_t rc = process_vm_readv(getpid(), &dst_io, 1, &src_io, 1, 0);
+  if (rc != sizeof(ehdr)) {
+    return 0;
+  }
+
+  uintptr_t addr = start_ + ehdr.e_phoff;
   for (size_t i = 0; i < ehdr.e_phnum; i++) {
     ElfW(Phdr) phdr;
-    if (!get_val<ElfW(Word)>(entry, addr + offsetof(ElfW(Phdr), p_type), &phdr.p_type)) {
-      return;
-    }
-    if (!get_val<ElfW(Word)>(entry, addr + offsetof(ElfW(Phdr), p_flags), &phdr.p_flags)) {
-      return;
-    }
-    if (!get_val<ElfW(Off)>(entry, addr + offsetof(ElfW(Phdr), p_offset), &phdr.p_offset)) {
-      return;
+
+    src_io.iov_base = reinterpret_cast<void*>(addr);
+    src_io.iov_len = sizeof(phdr);
+    dst_io.iov_base = &phdr;
+    dst_io.iov_len = sizeof(phdr);
+    rc = process_vm_readv(getpid(), &dst_io, 1, &src_io, 1, 0);
+    if (rc != sizeof(phdr)) {
+      return 0;
     }
     if ((phdr.p_type == PT_LOAD) && (phdr.p_flags & PF_X) ) {
-      if (!get_val<ElfW(Addr)>(entry, addr + offsetof(ElfW(Phdr), p_vaddr), &phdr.p_vaddr)) {
-        return;
-      }
-      entry->load_bias = phdr.p_vaddr - phdr.p_offset;
-      return;
+      load_bias_ = phdr.p_vaddr - phdr.p_offset;
+      return load_bias_;
     }
     addr += sizeof(phdr);
   }
+  return 0;
 }
 
-static void inline init(MapEntry* entry) {
-  if (entry->init) {
-    return;
-  }
-  entry->init = true;
-  if (valid_elf(entry)) {
-    entry->valid = true;
-    read_loadbias(entry);
-  }
-}
-
-bool MapData::ReadMaps() {
+void MapData::ReadMaps() {
+  std::lock_guard<std::mutex> lock(m_);
   FILE* fp = fopen("/proc/self/maps", "re");
   if (fp == nullptr) {
-    return false;
+    return;
   }
 
+  ClearEntries();
+
   std::vector<char> buffer(1024);
   while (fgets(buffer.data(), buffer.size(), fp) != nullptr) {
     MapEntry* entry = parse_line(buffer.data());
     if (entry == nullptr) {
-      fclose(fp);
-      return false;
+      break;
     }
-
-    auto it = entries_.find(entry);
-    if (it == entries_.end()) {
-      entries_.insert(entry);
-    } else {
-      delete entry;
-    }
+    entries_.insert(entry);
   }
   fclose(fp);
-  return true;
 }
 
-MapData::~MapData() {
+void MapData::ClearEntries() {
   for (auto* entry : entries_) {
     delete entry;
   }
   entries_.clear();
 }
 
+MapData::~MapData() {
+  ClearEntries();
+}
+
 // Find the containing map info for the PC.
 const MapEntry* MapData::find(uintptr_t pc, uintptr_t* rel_pc) {
   MapEntry pc_entry(pc);
 
   std::lock_guard<std::mutex> lock(m_);
-
   auto it = entries_.find(&pc_entry);
   if (it == entries_.end()) {
-    ReadMaps();
-  }
-  it = entries_.find(&pc_entry);
-  if (it == entries_.end()) {
     return nullptr;
   }
 
   MapEntry* entry = *it;
-  init(entry);
+  entry->Init();
 
   if (rel_pc != nullptr) {
     // Need to check to see if this is a read-execute map and the read-only
     // map is the previous one.
-    if (!entry->valid && it != entries_.begin()) {
+    if (!entry->valid() && it != entries_.begin()) {
       MapEntry* prev_entry = *--it;
-      if (prev_entry->flags == PROT_READ && prev_entry->offset < entry->offset &&
-          prev_entry->name == entry->name) {
-        init(prev_entry);
+      if (prev_entry->flags() == PROT_READ && prev_entry->offset() < entry->offset() &&
+          prev_entry->name() == entry->name()) {
+        prev_entry->Init();
 
-        if (prev_entry->valid) {
-          entry->elf_start_offset = prev_entry->offset;
-          *rel_pc = pc - entry->start + entry->offset + prev_entry->load_bias;
+        if (prev_entry->valid()) {
+          entry->set_elf_start_offset(prev_entry->offset());
+          *rel_pc = pc - entry->start() + entry->offset() + prev_entry->GetLoadBias();
           return entry;
         }
       }
     }
-    *rel_pc = pc - entry->start + entry->offset + entry->load_bias;
+    *rel_pc = pc - entry->start() + entry->offset() + entry->GetLoadBias();
   }
   return entry;
 }
diff --git a/libc/malloc_debug/MapData.h b/libc/malloc_debug/MapData.h
index f2b3c1c..13bf9cb 100644
--- a/libc/malloc_debug/MapData.h
+++ b/libc/malloc_debug/MapData.h
@@ -36,26 +36,50 @@
 
 #include <platform/bionic/macros.h>
 
-struct MapEntry {
-  MapEntry(uintptr_t start, uintptr_t end, uintptr_t offset, const char* name, size_t name_len, int flags)
-      : start(start), end(end), offset(offset), name(name, name_len), flags(flags) {}
+class MapEntry {
+ public:
+  MapEntry() = default;
+  MapEntry(uintptr_t start, uintptr_t end, uintptr_t offset, const char* name, size_t name_len,
+           int flags)
+      : start_(start), end_(end), offset_(offset), name_(name, name_len), flags_(flags) {}
 
-  explicit MapEntry(uintptr_t pc) : start(pc), end(pc) {}
+  explicit MapEntry(uintptr_t pc) : start_(pc), end_(pc) {}
 
-  uintptr_t start;
-  uintptr_t end;
-  uintptr_t offset;
-  uintptr_t load_bias;
-  uintptr_t elf_start_offset = 0;
-  std::string name;
-  int flags;
-  bool init = false;
-  bool valid = false;
+  void Init();
+
+  uintptr_t GetLoadBias();
+
+  void SetInvalid() {
+    valid_ = false;
+    init_ = true;
+    load_bias_read_ = true;
+  }
+
+  bool valid() { return valid_; }
+  uintptr_t start() const { return start_; }
+  uintptr_t end() const { return end_; }
+  uintptr_t offset() const { return offset_; }
+  uintptr_t elf_start_offset() const { return elf_start_offset_; }
+  void set_elf_start_offset(uintptr_t elf_start_offset) { elf_start_offset_ = elf_start_offset; }
+  const std::string& name() const { return name_; }
+  int flags() const { return flags_; }
+
+ private:
+  uintptr_t start_;
+  uintptr_t end_;
+  uintptr_t offset_;
+  uintptr_t load_bias_ = 0;
+  uintptr_t elf_start_offset_ = 0;
+  std::string name_;
+  int flags_;
+  bool init_ = false;
+  bool valid_ = false;
+  bool load_bias_read_ = false;
 };
 
 // Ordering comparator that returns equivalence for overlapping entries
 struct compare_entries {
-  bool operator()(const MapEntry* a, const MapEntry* b) const { return a->end <= b->start; }
+  bool operator()(const MapEntry* a, const MapEntry* b) const { return a->end() <= b->start(); }
 };
 
 class MapData {
@@ -65,11 +89,15 @@
 
   const MapEntry* find(uintptr_t pc, uintptr_t* rel_pc = nullptr);
 
- private:
-  bool ReadMaps();
+  size_t NumMaps() { return entries_.size(); }
 
+  void ReadMaps();
+
+ private:
   std::mutex m_;
   std::set<MapEntry*, compare_entries> entries_;
 
+  void ClearEntries();
+
   BIONIC_DISALLOW_COPY_AND_ASSIGN(MapData);
 };
diff --git a/libc/malloc_debug/backtrace.cpp b/libc/malloc_debug/backtrace.cpp
index ecb3a80..6a32fca 100644
--- a/libc/malloc_debug/backtrace.cpp
+++ b/libc/malloc_debug/backtrace.cpp
@@ -50,7 +50,7 @@
 typedef struct _Unwind_Context __unwind_context;
 
 static MapData g_map_data;
-static const MapEntry* g_current_code_map = nullptr;
+static MapEntry g_current_code_map;
 
 static _Unwind_Reason_Code find_current_map(__unwind_context* context, void*) {
   uintptr_t ip = _Unwind_GetIP(context);
@@ -58,11 +58,15 @@
   if (ip == 0) {
     return _URC_END_OF_STACK;
   }
-  g_current_code_map = g_map_data.find(ip);
+  auto map = g_map_data.find(ip);
+  if (map != nullptr) {
+    g_current_code_map = *map;
+  }
   return _URC_END_OF_STACK;
 }
 
 void backtrace_startup() {
+  g_map_data.ReadMaps();
   _Unwind_Backtrace(find_current_map, nullptr);
 }
 
@@ -98,7 +102,8 @@
   }
 
   // Do not record the frames that fall in our own shared library.
-  if (g_current_code_map && (ip >= g_current_code_map->start) && ip < g_current_code_map->end) {
+  if (g_current_code_map.start() != 0 && (ip >= g_current_code_map.start()) &&
+      ip < g_current_code_map.end()) {
     return _URC_NO_REASON;
   }
 
@@ -113,6 +118,10 @@
 }
 
 std::string backtrace_string(const uintptr_t* frames, size_t frame_count) {
+  if (g_map_data.NumMaps() == 0) {
+    g_map_data.ReadMaps();
+  }
+
   std::string str;
 
   for (size_t frame_num = 0; frame_num < frame_count; frame_num++) {
@@ -130,14 +139,15 @@
     uintptr_t rel_pc = offset;
     const MapEntry* entry = g_map_data.find(frames[frame_num], &rel_pc);
 
-    const char* soname = (entry != nullptr) ? entry->name.c_str() : info.dli_fname;
+    const char* soname = (entry != nullptr) ? entry->name().c_str() : info.dli_fname;
     if (soname == nullptr) {
       soname = "<unknown>";
     }
 
     char offset_buf[128];
-    if (entry != nullptr && entry->elf_start_offset != 0) {
-      snprintf(offset_buf, sizeof(offset_buf), " (offset 0x%" PRIxPTR ")", entry->elf_start_offset);
+    if (entry != nullptr && entry->elf_start_offset() != 0) {
+      snprintf(offset_buf, sizeof(offset_buf), " (offset 0x%" PRIxPTR ")",
+               entry->elf_start_offset());
     } else {
       offset_buf[0] = '\0';
     }
@@ -167,5 +177,6 @@
 }
 
 void backtrace_log(const uintptr_t* frames, size_t frame_count) {
+  g_map_data.ReadMaps();
   error_log_string(backtrace_string(frames, frame_count).c_str());
 }