Re: [PATCH v2] Allow shrinking of arena heaps using mmap based onovercommit settings when available

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

Hi Roland,

Here is an updated patch with further fixes as per your review. No
regressions in the testsuite. Also, I have tested this on F16 x86_64 as
follows:

* echo 0 > /proc/sys/vm/overcommit_memory
* Run the benchmark program that I posted earlier:

http://sourceware.org/ml/libc-alpha/2012-08/msg00462.html

and attach an strace as follows:

strace -f -e madvise -p $(pgrep a.out)

to confirm that madvise is being called. Detach and attach strace again
as follows:

strace -f -e mmap -p $(pgrep a.out)

to ensure that there are no mmap calls with MAP_FIXED.

Now disable overcommit:

* echo 2 > /proc/sys/vm/overcommit_memory
* Run the benchmark program and verify that the behaviour is the
  opposite, i.e. there should be no calls to madvise and you should see
  calls to mmap with MAP_FIXED.

Further, I also verified that I see the performance difference between
madvise and MAP_FIXED:

with overcommit_memory == 0:

RUNS=4096, TOTAL=8859615.000000, AVG=2162.991943
RUNS=4096, TOTAL=8948967.000000, AVG=2184.806396
RUNS=4096, TOTAL=9002337.000000, AVG=2197.836182
RUNS=4096, TOTAL=9147284.000000, AVG=2233.223633
RUNS=4096, TOTAL=8852469.000000, AVG=2161.247314
RUNS=4096, TOTAL=8920974.000000, AVG=2177.972168
RUNS=4096, TOTAL=9051495.000000, AVG=2209.837646
RUNS=4096, TOTAL=8896417.000000, AVG=2171.976807

with overcommit_memory == 2:

RUNS=4096, TOTAL=9916217.000000, AVG=2420.951416
RUNS=4096, TOTAL=10098292.000000, AVG=2465.403320
RUNS=4096, TOTAL=9951147.000000, AVG=2429.479248
RUNS=4096, TOTAL=9776165.000000, AVG=2386.759033
RUNS=4096, TOTAL=9735272.000000, AVG=2376.775391
RUNS=4096, TOTAL=10049977.000000, AVG=2453.607666
RUNS=4096, TOTAL=9967350.000000, AVG=2433.435059
RUNS=4096, TOTAL=9817866.000000, AVG=2396.939941

Regards,
Siddhesh

ChangeLog:

	* malloc/arena.c: Include malloc-sysdep.h.
	(shrink_heap): Use check_may_shrink_heap to decide if madvise
	is sufficient to shrink the heap or an unmap is needed.
	* sysdeps/generic/malloc-sysdep.h: New file.  Define
	new function check_may_shrink_heap.
	* sysdeps/unix/sysv/linux/malloc-sysdep.h: New file.  Define
	new function check_may_shrink_heap.

diff --git a/malloc/arena.c b/malloc/arena.c
index 97c0b90..50f3f96 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -19,6 +19,9 @@
 
 #include <stdbool.h>
 
+/* Get the implementation for check_may_shrink_heap.  */
+#include <malloc-sysdep.h>
+
 /* Compile-time constants.  */
 
 #define HEAP_MIN_SIZE (32*1024)
@@ -621,9 +624,9 @@ shrink_heap(heap_info *h, long diff)
   new_size = (long)h->size - diff;
   if(new_size < (long)sizeof(*h))
     return -1;
-  /* Try to re-map the extra heap space freshly to save memory, and
-     make it inaccessible. */
-  if (__builtin_expect (__libc_enable_secure, 0))
+  /* Try to re-map the extra heap space freshly to save memory, and make it
+     inaccessible.  See malloc-sysdep.h to know when this is true.  */
+  if (__builtin_expect (check_may_shrink_heap (), 0))
     {
       if((char *)MMAP((char *)h + new_size, diff, PROT_NONE,
 		      MAP_FIXED) == (char *) MAP_FAILED)
diff --git a/sysdeps/generic/malloc-sysdep.h b/sysdeps/generic/malloc-sysdep.h
new file mode 100644
index 0000000..bbc48c0
--- /dev/null
+++ b/sysdeps/generic/malloc-sysdep.h
@@ -0,0 +1,25 @@
+/* System-specific malloc support functions.  Generic version.
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* Force an unmap when the heap shrinks in a secure exec.  This ensures that
+   the old data pages immediately cease to be accessible.  */
+static inline bool
+check_may_shrink_heap (void)
+{
+  return __libc_enable_secure;
+}
diff --git a/sysdeps/unix/sysv/linux/malloc-sysdep.h b/sysdeps/unix/sysv/linux/malloc-sysdep.h
new file mode 100644
index 0000000..f926aea
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/malloc-sysdep.h
@@ -0,0 +1,57 @@
+/* System-specific malloc support functions.  Linux version.
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <fcntl.h>
+#include <not-cancel.h>
+
+/* The Linux kernel overcommits address space by default and if there is not
+   enough memory available, it uses various parameters to decide the process to
+   kill.  It is however possible to disable or curb this overcommit behavior
+   by setting the proc sysctl vm.overcommit_memory to the value '2' and with
+   that, a process is only allowed to use the maximum of a pre-determined
+   fraction of the total address space.  In such a case, we want to make sure
+   that we are judicious with our heap usage as well, and explicitly give away
+   the freed top of the heap to reduce our commit charge.  See the proc(5) man
+   page to know more about overcommit behavior.
+
+   Other than that, we also force an unmap in a secure exec.  */
+static inline bool
+check_may_shrink_heap (void)
+{
+  static int may_shrink_heap = -1;
+
+  if (__builtin_expect (may_shrink_heap >= 0, 1))
+    return may_shrink_heap;
+
+  may_shrink_heap = __libc_enable_secure;
+
+  if (__builtin_expect (may_shrink_heap == 0, 1))
+    {
+      int fd = open_not_cancel_2 ("/proc/sys/vm/overcommit_memory",
+				  O_RDONLY | O_CLOEXEC);
+      if (fd >= 0)
+	{
+	  char val;
+	  ssize_t n = read_not_cancel (fd, &val, 1);
+	  may_shrink_heap = n > 0 && val == '2';
+	  close_not_cancel_no_status (fd);
+	}
+    }
+
+  return may_shrink_heap;
+}