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");
 }
(<a href='/guix/log/tests/elm.scm?id=337d2d0f9f36d9ff0fc6a2014f20738997f680ac&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-09-17 17:34:58 +0200'>2024-09-17</span></td><td><a href='/guix/commit/tests/elm.scm?id=7979001c58fcadd98bae09781c495f0a462d6e04'>tests: Adjust ‘elm’ importer test to expect inputs without labels.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup af85c38b017be3d932d4665acd9ff4b5c56a9790.

* tests/elm.scm ("(guix import elm)"): Expect new-style inputs.

Change-Id: If3c6b17b5a5d17ad443c3ddb05f94dc2458ba182
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2024-06-03 22:58:53 +0200'>2024-06-03</span></td><td><a href='/guix/commit/tests/elm.scm?id=510ad5d93c0ba6208aa42815863853ec522352e6'>import: utils: End package descriptions with period.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/utils.scm (beautify-description): Append period to last
words which do not end with one.
* tests/crate.scm: Append period to descriptions.
* tests/elm.scm: Append period to descriptions.
* tests/gem.scm: Append period to descriptions.
* tests/hexpm.scm: Append period to descriptions.
* tests/minetest.scm: Append period to descriptions.
* tests/pypi.scm: Append period to descriptions.
* tests/import-utils.scm ("beautify-description: transform fragment into
sentence"): Likewise.

Change-Id: I0b12c4d94cb26cf62fab5b7cbf7885e66ff6c10f
Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Herman Rimm</td></tr>
<tr><td><span title='2022-05-22 01:07:52 +0200'>2022-05-22</span></td><td><a href='/guix/commit/tests/elm.scm?id=903c82583e1cec4c9ff09d5895c5cc646c37b661'>import: Add Elm importer.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/elm.scm, guix/scripts/import/elm.scm: New files.
* Makefile.am (MODULES): Add them.
* guix/scripts/import.scm (importers): Add "elm".
* doc/guix.texi (Invoking guix import): Document Elm importer.
* doc/contributing.texi (Elm Packages): Mention it.
* tests/elm.scm ("(guix import elm)"): New test group.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Philip McGrath</td></tr>
<tr><td><span title='2022-05-22 01:07:51 +0200'>2022-05-22</span></td><td><a href='/guix/commit/tests/elm.scm?id=aefcfdd0ae2aa41adb3d1106752b6a291ef1d80f'>guix: Add elm-build-system.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/patches/elm-offline-package-registry.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/elm.scm (elm): Use it.
* guix/build-system/elm.scm, guix/build/elm-build-system.scm,
tests/elm.scm: New files.
* Makefile.scm (MODULES, SCM_TESTS): Add them.
* doc/guix.texi (Build Systems): Document 'elm-build-system'.
* doc/contributing.texi (Elm Packages): New section. Document naming
conventions and utilities.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Philip McGrath</td></tr>