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


Hi,

Here's an updated patch with changes according to Roland's review
comments.  This works fine on my x86_64 Fedora 16 laptop and does not
cause regressions in the testsuite.

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 9e5e332..480491b 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -20,6 +20,9 @@
 
 #include <stdbool.h>
 
+/* Get the implementation for check_may_shrink_heap.  */
+#include <malloc-sysdep.h>
+
 /* Compile-time constants.  */
 
 #define HEAP_MIN_SIZE (32*1024)
@@ -622,9 +625,9 @@ 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..4ea85aa
--- /dev/null
+++ b/sysdeps/generic/malloc-sysdep.h
@@ -0,0 +1,28 @@
+/* System-specific implementations for some malloc support functions.
+
+   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 <unistd.h>
+
+/* Force an unmap on heap shrink for setuid programs.  This is so that the
+   kernel immediately drops the PTE for the mapping.  */
+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..be5c6d3
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/malloc-sysdep.h
@@ -0,0 +1,61 @@
+/* System-specific implementations for some malloc support functions.
+
+   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 <unistd.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 behaviour
+   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 behaviour.
+
+   Other than that, we also force an unmap for setuid programs.  */
+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))
+    return may_shrink_heap;
+
+  int fd = open_not_cancel_2 ("/proc/sys/vm/overcommit_memory",
+			      O_RDONLY | O_CLOEXEC);
+
+  if (fd != -1)
+    {
+      char val = 0;
+      int n = read_not_cancel (fd, &val, 1);
+
+      may_shrink_heap = (n > 0 && val == 2);
+      close_not_cancel_no_status (fd);
+    }
+
+  return may_shrink_heap;
+}

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