Index: compiler-rt/lib/hwasan/hwasan.cc
===================================================================
--- compiler-rt/lib/hwasan/hwasan.cc
+++ compiler-rt/lib/hwasan/hwasan.cc
@@ -88,6 +88,8 @@
     cf.check_printf = false;
     cf.intercept_tls_get_addr = true;
     cf.exitcode = 99;
+    // 8 shadow pages ~~ 512kB, small enough to cover common stack sizes.
+    cf.clear_shadow_mmap_threshold = 4096 * (SANITIZER_ANDROID ? 2 : 8);
     // Sigtrap is used in error reporting.
     cf.handle_sigtrap = kHandleSignalExclusive;
 
Index: compiler-rt/lib/hwasan/hwasan_poisoning.cc
===================================================================
--- compiler-rt/lib/hwasan/hwasan_poisoning.cc
+++ compiler-rt/lib/hwasan/hwasan_poisoning.cc
@@ -24,7 +24,24 @@
   CHECK(IsAligned(size, kShadowAlignment));
   uptr shadow_start = MemToShadow(p);
   uptr shadow_size = MemToShadowSize(size);
-  internal_memset((void *)shadow_start, tag, shadow_size);
+
+  uptr page_size = GetPageSizeCached();
+  uptr page_start = RoundUpTo(shadow_start, page_size);
+  uptr page_end = RoundDownTo(shadow_start + shadow_size, page_size);
+  uptr threshold = common_flags()->clear_shadow_mmap_threshold;
+  if (SANITIZER_LINUX &&
+      UNLIKELY(page_end >= page_start + threshold && tag == 0)) {
+    if (page_start > shadow_start)
+      internal_memset((void *)shadow_start, tag, page_start - shadow_start);
+    if (shadow_start + shadow_size > page_end)
+      internal_memset((void *)page_end, tag,
+                      shadow_start + shadow_size - page_end);
+    // For an anonymous private mapping MADV_DONTNEED will return a zero page on
+    // Linux.
+    ReleaseMemoryPagesToOS(page_start, page_end);
+  } else {
+    internal_memset((void *)shadow_start, tag, shadow_size);
+  }
   return AddTagToPointer(p, tag);
 }