Gracefully handle MADV_FREE available at compile time but not run time

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 9678488e340e30e78daf86f88a1601b51fd81c0d..5b1ef8ba2f8b1993be61ab7565b1122f5003d47f 100644
--- a/base/allocator/partition_allocator/page_allocator.cc
+++ b/base/allocator/partition_allocator/page_allocator.cc
@@ -216,6 +216,12 @@ void decommitSystemPages(void* addr, size_t len) {
   DCHECK(!(len & kSystemPageOffsetMask));
 #if defined(OS_POSIX)
   int ret = madvise(addr, len, MADV_FREE);
+  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(addr, len, MADV_DONTNEED);
+  }
   CHECK(!ret);
 #else
   setSystemPagesInaccessible(addr, len);