Index: compiler-rt/trunk/lib/asan/asan_poisoning.h
===================================================================
--- compiler-rt/trunk/lib/asan/asan_poisoning.h
+++ compiler-rt/trunk/lib/asan/asan_poisoning.h
@@ -86,8 +86,8 @@
   }
 }
 
-// Calls __sanitizer::ReleaseMemoryToOS() on
-// [MemToShadow(p), MemToShadow(p+size)] with proper rounding.
+// Calls __sanitizer::ReleaseMemoryPagesToOS() on
+// [MemToShadow(p), MemToShadow(p+size)].
 void FlushUnneededASanShadowMemory(uptr p, uptr size);
 
 }  // namespace __asan
Index: compiler-rt/trunk/lib/asan/asan_poisoning.cc
===================================================================
--- compiler-rt/trunk/lib/asan/asan_poisoning.cc
+++ compiler-rt/trunk/lib/asan/asan_poisoning.cc
@@ -64,12 +64,9 @@
 };
 
 void FlushUnneededASanShadowMemory(uptr p, uptr size) {
-    // Since asan's mapping is compacting, the shadow chunk may be
-    // not page-aligned, so we only flush the page-aligned portion.
-    uptr page_size = GetPageSizeCached();
-    uptr shadow_beg = RoundUpTo(MemToShadow(p), page_size);
-    uptr shadow_end = RoundDownTo(MemToShadow(p + size), page_size);
-    ReleaseMemoryToOS(shadow_beg, shadow_end - shadow_beg);
+  // Since asan's mapping is compacting, the shadow chunk may be
+  // not page-aligned, so we only flush the page-aligned portion.
+  ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
 }
 
 void AsanPoisonOrUnpoisonIntraObjectRedzone(uptr ptr, uptr size, bool poison) {
Index: compiler-rt/trunk/lib/msan/msan_allocator.cc
===================================================================
--- compiler-rt/trunk/lib/msan/msan_allocator.cc
+++ compiler-rt/trunk/lib/msan/msan_allocator.cc
@@ -33,9 +33,12 @@
 
     // We are about to unmap a chunk of user memory.
     // Mark the corresponding shadow memory as not needed.
-    ReleaseMemoryToOS(MEM_TO_SHADOW(p), size);
-    if (__msan_get_track_origins())
-      ReleaseMemoryToOS(MEM_TO_ORIGIN(p), size);
+    uptr shadow_p = MEM_TO_SHADOW(p);
+    ReleaseMemoryPagesToOS(shadow_p, shadow_p + size);
+    if (__msan_get_track_origins()) {
+      uptr origin_p = MEM_TO_ORIGIN(p);
+      ReleaseMemoryPagesToOS(origin_p, origin_p + size);
+    }
   }
 };
 
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_allocator_primary64.h
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_allocator_primary64.h
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_allocator_primary64.h
@@ -458,17 +458,11 @@
     }
   }
 
-  bool MaybeReleaseChunkRange(uptr region_beg, uptr chunk_size,
+  void MaybeReleaseChunkRange(uptr region_beg, uptr chunk_size,
                               CompactPtrT first, CompactPtrT last) {
     uptr beg_ptr = CompactPtrToPointer(region_beg, first);
     uptr end_ptr = CompactPtrToPointer(region_beg, last) + chunk_size;
-    const uptr page_size = GetPageSizeCached();
-    CHECK_GE(end_ptr - beg_ptr, page_size);
-    beg_ptr = RoundUpTo(beg_ptr, page_size);
-    end_ptr = RoundDownTo(end_ptr, page_size);
-    if (end_ptr == beg_ptr) return false;
-    ReleaseMemoryToOS(beg_ptr, end_ptr - beg_ptr);
-    return true;
+    ReleaseMemoryPagesToOS(beg_ptr, end_ptr);
   }
 
   // Attempts to release some RAM back to OS. The region is expected to be
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_common.h
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_common.h
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_common.h
@@ -103,7 +103,9 @@
 
 // Used to check if we can map shadow memory to a fixed location.
 bool MemoryRangeIsAvailable(uptr range_start, uptr range_end);
-void ReleaseMemoryToOS(uptr addr, uptr size);
+// Releases memory pages entirely within the [beg, end] address range. Noop if
+// the provided range does not contain at least one entire page.
+void ReleaseMemoryPagesToOS(uptr beg, uptr end);
 void IncreaseTotalMmap(uptr size);
 void DecreaseTotalMmap(uptr size);
 uptr GetRSS();
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_posix_libcdep.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_posix_libcdep.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_posix_libcdep.cc
@@ -56,8 +56,12 @@
   return (uptr)pthread_self();
 }
 
-void ReleaseMemoryToOS(uptr addr, uptr size) {
-  madvise((void*)addr, size, MADV_DONTNEED);
+void ReleaseMemoryPagesToOS(uptr beg, uptr end) {
+  uptr page_size = GetPageSizeCached();
+  uptr beg_aligned = RoundUpTo(beg, page_size);
+  uptr end_aligned = RoundDownTo(end, page_size);
+  if (beg_aligned < end_aligned)
+    madvise((void*)beg_aligned, end_aligned - beg_aligned, MADV_DONTNEED);
 }
 
 void NoHugePagesInRegion(uptr addr, uptr size) {
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_win.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_win.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_win.cc
@@ -234,8 +234,7 @@
   return VirtualProtect((LPVOID)addr, size, PAGE_NOACCESS, &old_protection);
 }
 
-
-void ReleaseMemoryToOS(uptr addr, uptr size) {
+void ReleaseMemoryPagesToOS(uptr beg, uptr end) {
   // This is almost useless on 32-bits.
   // FIXME: add madvise-analog when we move to 64-bits.
 }
Index: compiler-rt/trunk/lib/tsan/rtl/tsan_mman.cc
===================================================================
--- compiler-rt/trunk/lib/tsan/rtl/tsan_mman.cc
+++ compiler-rt/trunk/lib/tsan/rtl/tsan_mman.cc
@@ -54,7 +54,8 @@
     diff = p + size - RoundDown(p + size, kPageSize);
     if (diff != 0)
       size -= diff;
-    ReleaseMemoryToOS((uptr)MemToMeta(p), size / kMetaRatio);
+    uptr p_meta = (uptr)MemToMeta(p);
+    ReleaseMemoryPagesToOS(p_meta, p_meta + size / kMetaRatio);
   }
 };
 
Index: compiler-rt/trunk/lib/tsan/rtl/tsan_platform_linux.cc
===================================================================
--- compiler-rt/trunk/lib/tsan/rtl/tsan_platform_linux.cc
+++ compiler-rt/trunk/lib/tsan/rtl/tsan_platform_linux.cc
@@ -134,7 +134,7 @@
 void FlushShadowMemoryCallback(
     const SuspendedThreadsList &suspended_threads_list,
     void *argument) {
-  ReleaseMemoryToOS(ShadowBeg(), ShadowEnd() - ShadowBeg());
+  ReleaseMemoryPagesToOS(ShadowBeg(), ShadowEnd());
 }
 #endif
 
Index: compiler-rt/trunk/lib/tsan/rtl/tsan_rtl.cc
===================================================================
--- compiler-rt/trunk/lib/tsan/rtl/tsan_rtl.cc
+++ compiler-rt/trunk/lib/tsan/rtl/tsan_rtl.cc
@@ -234,9 +234,7 @@
 #endif
 
 void DontNeedShadowFor(uptr addr, uptr size) {
-  uptr shadow_beg = MemToShadow(addr);
-  uptr shadow_end = MemToShadow(addr + size);
-  ReleaseMemoryToOS(shadow_beg, shadow_end - shadow_beg);
+  ReleaseMemoryPagesToOS(MemToShadow(addr), MemToShadow(addr + size));
 }
 
 void MapShadow(uptr addr, uptr size) {
Index: compiler-rt/trunk/lib/tsan/rtl/tsan_rtl_thread.cc
===================================================================
--- compiler-rt/trunk/lib/tsan/rtl/tsan_rtl_thread.cc
+++ compiler-rt/trunk/lib/tsan/rtl/tsan_rtl_thread.cc
@@ -68,7 +68,8 @@
 
 void ThreadContext::OnReset() {
   CHECK_EQ(sync.size(), 0);
-  ReleaseMemoryToOS(GetThreadTrace(tid), TraceSize() * sizeof(Event));
+  uptr trace_p = GetThreadTrace(tid);
+  ReleaseMemoryPagesToOS(trace_p, trace_p + TraceSize() * sizeof(Event));
   //!!! ReleaseMemoryToOS(GetThreadTraceHeader(tid), sizeof(Trace));
 }