[commit: ghc] master: rts: Ensure we always give MADV_DONTNEED a chance in osDecommitMemory (6576bf8)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6576bf83cdf4eac05eb88a24aa934a736c91e3da/ghc

>---------------------------------------------------------------

commit 6576bf83cdf4eac05eb88a24aa934a736c91e3da
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Thu Dec 1 12:55:23 2016 -0500

    rts: Ensure we always give MADV_DONTNEED a chance in osDecommitMemory
    
    As described in #12865, newer Linux kernels support both MADV_FREE and
    MADV_DONTNEED. Previously a runtime would fail to try MADV_DONTNEED if
    MADV_FREE failed (e.g. since the kernel which the image is running on
    doesn't support the latter). Now we try MADV_DONTNEED if MADV_FREE
    failed to ensure that binaries compiled on a kernel supporting MADV_FREE
    don't fail on decommit.
    
    Test Plan: Validate
    
    Reviewers: austin, erikd, simonmar
    
    Reviewed By: simonmar
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2780
    
    GHC Trac Issues: #12865


>---------------------------------------------------------------

6576bf83cdf4eac05eb88a24aa934a736c91e3da
 rts/posix/OSMem.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/rts/posix/OSMem.c b/rts/posix/OSMem.c
index 5291745..beffeda 100644
--- 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");
 }