Add some support for promptly purging pages.

base/allocator/partition_allocator/page_allocator.cc

Index: base/allocator/partition_allocator/page_allocator.cc
diff --git a/base/allocator/partition_allocator/page_allocator.cc b/base/allocator/partition_allocator/page_allocator.cc
index bb737b3341986d73739a2f5bed84d0c33052b1ce..1dba6d319f4a543c42f7ff02108e3cf01ec17009 100644
--- a/base/allocator/partition_allocator/page_allocator.cc
+++ b/base/allocator/partition_allocator/page_allocator.cc
@@ -213,8 +213,9 @@ bool SetSystemPagesAccessible(void* address, size_t length) {
 #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
@@ -222,7 +223,21 @@ void DecommitSystemPages(void* address, size_t length) {
   // 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,