ghc runtime by default (otherwise depending on a "configure" option)
does memory allocation on their own by first mmapping a 1 TB range of
memory into the process and then parceling out chunks from it.

If one of the chunks is not needed, the kernel needs to be informed -
otherwise the system would quickly run out of available RAM.

ghc does that via madvise(2).

There are two options when doing this informing:

MADV_FREE - Means "I don't need this range or the data in it any more". 
Kernel promises to fail later accesses to it.

MADV_DONTNEED - Means "I don't need this range right now - and I don't
need the data in it anymore". Kernel promises to make later accesses to
it succeed (if necessary by providing a new page initialized with zeroes).

MADV_FREE was introduced in Linux 4.5.
glibc 2.25 and later always define MADV_FREE.

Unpatched ghc 8.0.2 will use either MADV_FREE or MADV_DONTNEED, determined
at ghc compile time.  Which of them will actually succeed is determined
by the Linux kernel at run time.

This patch makes ghc try MADV_FREE.  If it doesn't work, it falls back to
MADV_DONTNEED.

The end result is that ghc programs free their memory with Linux < 4.5 again.

See https://git.haskell.org/ghc.git/commitdiff/6576bf83cdf4eac05eb88a24aa934a736c91e3da for more information.
--- a/rts/posix/OSMem.c
+++ b/rts/posix/OSMem.c
@@ -541,11 +541,24 @@ void osDecommitMemory(void *at, W_ size)
 
 #ifdef MADV_FREE
     // Try MADV_FREE first, FreeBSD has both and MADV_DONTNEED
-    // just swaps memory out
+    // just swaps memory out. Linux >= 4.5 has both DONTNEED and FREE; either
+    // will work as they both allow the system to free anonymous pages.
+    // It is important that we try both methods as the kernel which we were
+    // built on may differ from the kernel we are now running on.
     r = madvise(at, size, MADV_FREE);
-#else
-    r = madvise(at, size, MADV_DONTNEED);
+    if(r < 0) {
+        if (errno == EINVAL) {
+            // Perhaps the system doesn't support MADV_FREE; fall-through and
+            // try MADV_DONTNEED.
+        } else {
+            sysErrorBelch("unable to decommit memory");
+        }
+    } else {
+        return;
+    }
 #endif
+
+    r = madvise(at, size, MADV_DONTNEED);
     if(r < 0)
         sysErrorBelch("unable to decommit memory");
 }
280cd7deee5c58e11cd4'>download.in</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/nix/scripts/download.in?id=4f889375f2e3c0606e83280cd7deee5c58e11cd4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2016-11-16 18:19:47 +0100'>2016-11-16</span></td><td><a href='/guix/commit/nix/scripts/download.in?id=94d92c7796a3dd50c27d532315f7d497ac99f08e'>daemon: Add "builtin:download" derivation builder.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>