Add some support for promptly purging pages.

base/allocator/partition_allocator/page_allocator.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/allocator/partition_allocator/page_allocator.h"
 
 #include <limits.h>
 
 #include <atomic>
 
@@ skipping 195 matching lines @@
 
 bool SetSystemPagesAccessible(void* address, size_t length) {
   DCHECK(!(length & kSystemPageOffsetMask));
 #if defined(OS_POSIX)
   return !mprotect(address, length, PROT_READ | PROT_WRITE);
 #else
   return !!VirtualAlloc(address, length, MEM_COMMIT, PAGE_READWRITE);
 #endif
 }
 
-void DecommitSystemPages(void* address, size_t length) {
+void DecommitSystemPages(void* address, size_t length, bool purge_hint) {
   DCHECK(!(length & kSystemPageOffsetMask));
+  ALLOW_UNUSED_LOCAL(purge_hint);
 #if defined(OS_POSIX)
 #if defined(OS_MACOSX)
   // On macOS, MADV_FREE_REUSABLE has comparable behavior to MADV_FREE, but also
   // marks the pages with the reusable bit, which allows both Activity Monitor
   // and memory-infra to correctly track the pages.
   int ret = madvise(address, length, MADV_FREE_REUSABLE);
 #else
-  int ret = madvise(address, length, MADV_FREE);
+  int advice = MADV_FREE;
+// For platforms known to support MADV_REMOVE, use it to promptly
+// let go of the pages. |purge_hint| is otherwise disregarded, leaving
+// it to the OS' policy for recycling MADV_FREE'd pages.
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  if (purge_hint)
+    advice = MADV_REMOVE;
+#endif
+  int ret = madvise(address, length, advice);
+#if defined(OS_LINUX) || defined(OS_ANDROID)
+  // If MADV_REMOVE's zero'ing fails due to not being writable,
+  // fall back to freeing.
+  if (ret != 0 && purge_hint)
+    ret = madvise(address, length, MADV_FREE);
+#endif
 #endif
   if (ret != 0 && errno == EINVAL) {
     // MADV_FREE only works on Linux 4.5+ . If request failed,
     // retry with older MADV_DONTNEED . Note that MADV_FREE
     // being defined at compile time doesn't imply runtime support.
     ret = madvise(address, length, MADV_DONTNEED);
   }
   CHECK(!ret);
 #else
   SetSystemPagesInaccessible(address, length);
@@ skipping 41 matching lines @@
     CHECK(ret);
   }
 #endif
 }
 
 uint32_t GetAllocPageErrorCode() {
   return s_allocPageErrorCode;
 }
 
 }  // namespace base