From f617efdcac628c4bd8f4fc51b0475095b3dfec76 Mon Sep 17 00:00:00 2001
From: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>
Date: Tue, 9 Jul 2024 15:15:35 +0200
Subject: [PATCH] Add memtool program to test the memory

---
 JOURNAL.md     |   5 +++
 lagarto-ox.nix |   3 +-
 memtool.c      | 119 +++++++++++++++++++++++++++++++++++++++++++++++++
 overlay.nix    |  16 +++++++
 4 files changed, 142 insertions(+), 1 deletion(-)
 create mode 100644 memtool.c

diff --git a/JOURNAL.md b/JOURNAL.md
index 6080c78..54e6c2f 100644
--- a/JOURNAL.md
+++ b/JOURNAL.md
@@ -966,3 +966,8 @@ And it hangs just after exiting the tool.
 If the problem that we are observing is somehow related to the recursive
 segfault of the kernel in Cincoranch, we may be able to see the printk ring
 buffer by directly poking at the memory from the host.
+
+### QUESTION: Can we crash the CPU by exercising the memory?
+
+I did a small tool `memtool` that performs allocations and
+deallocations.
diff --git a/lagarto-ox.nix b/lagarto-ox.nix
index 4a750a1..4b3be5d 100644
--- a/lagarto-ox.nix
+++ b/lagarto-ox.nix
@@ -81,9 +81,10 @@
       kernelModules = [ ];
 
       # Add the csrtool to the initrd so we can change the
-      # in-order/out-of-order.
+      # in-order/out-of-order, and memtool to stress the memory.
       extraUtilsCommands = ''
         cp -a ${pkgs.csrtool}/bin/csrtool $out/bin
+        cp -a ${pkgs.memtool}/bin/memtool $out/bin
       '';
 
       # Write a counter to the DMA region, so we can check the kernel is not
diff --git a/memtool.c b/memtool.c
new file mode 100644
index 0000000..b5b6f2a
--- /dev/null
+++ b/memtool.c
@@ -0,0 +1,119 @@
+/* Copyright (c) 2024 Barcelona Supercomputing Center (BSC)
+ * SPDX-License-Identifier: MIT
+ * Author: Rodrigo Arias Mallo <rodrigo.arias@bsc.es> */
+
+/* This is just a small tool to exercise the memory which attempts to
+ * stress the virtual memory, in a crude attempt to reproduce the hangs
+ * that we were observing while booting NixOS. */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+
+#define MAX_SIZE (1024L * 1024L)
+
+struct block {
+	struct block *next;
+	size_t size;
+	uint32_t data[];
+};
+
+struct block *front = NULL;
+struct block *tail = NULL;
+long nblocks = 0;
+long nbytes = 0;
+
+static int
+allocate(void)
+{
+	long n = (long) rand() % MAX_SIZE;
+	/* Constraint the size */
+	n = n % MAX_SIZE;
+
+	size_t size = sizeof(struct block) + n * sizeof(uint32_t);
+
+	struct block *b = malloc(size);
+
+	/* No mem */
+	if (b == NULL)
+		return -1;
+
+	b->size = size;
+	b->next = NULL;
+
+	/* Populate the block with some data */
+	for (long i = 0; i < n; i++)
+		b->data[i] = rand();
+
+	/* Add it to the chain */
+	if (tail)
+		tail->next = b;
+
+	tail = b;
+
+	/* And to the front if it is the first */
+	if (!front)
+		front = b;
+
+	nblocks++;
+	nbytes += size;
+
+	return 0;
+}
+
+static int
+deallocate(void)
+{
+	/* May run out of blocks */
+	if (!front)
+		return -1;
+
+	struct block *b = front;
+
+	front = b->next;
+
+	/* Last block */
+	if (tail == b)
+		tail = NULL;
+
+	nblocks--;
+	nbytes -= b->size;
+
+	free(b);
+
+	return 0;
+}
+
+static void
+torture(void)
+{
+	srand(123);
+
+	for (long iter = 0; ; iter++) {
+		int p = rand() % 100;
+		int is_alloc = (p > 10);
+		int ret = 0;
+		char c;
+		if (is_alloc) {
+			if (allocate() == 0)
+				c = 'A';
+			else
+				c = '-';
+		} else {
+			if (deallocate() == 0)
+				c = 'D';
+			else
+				c = '-';
+		}
+
+		printf("iter %ld, nblocks %ld, nbytes %.1fM (%c)\n",
+				iter, nblocks, (double) nbytes / (1024. * 1024.),
+				c);
+	}
+}
+
+int main(int argc, char *argv[])
+{
+	torture();
+	return 0;
+}
diff --git a/overlay.nix b/overlay.nix
index 1614a9a..5813639 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -53,6 +53,22 @@ final: prev:
     '';
   };
 
+  memtool = prev.pkgsStatic.stdenv.mkDerivation {
+    name = "memtool";
+    src = ./memtool.c;
+    unpackPhase = ''
+      cp ${./memtool.c} memtool.c
+    '';
+    dontConfigure = true;
+    buildPhase = ''
+      $CC -static memtool.c -o memtool
+    '';
+    installPhase = ''
+      mkdir -p $out/bin
+      cp memtool $out/bin/
+    '';
+  };
+
   bitstreams = builtins.fetchGit {
     url = "git@bscpm03.bsc.es:rarias/bitstreams.git";
     rev = "ad901b0c21ffbdb310ff1dfb269f169f6ac6bde6";