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Re: [PATCH v2] Allow shrinking of arena heaps using mmap based onovercommit settings when available
On Wed, 22 Aug 2012 00:06:10 +0530, Siddhesh wrote:
> ChangeLog:
>
> * malloc/arena.c: Include malloc-sysdep.h.
> (shrink_heap): New static variable may_shrink_heap to decide
> if madvise is sufficient to shrink the heap or an unmap is needed.
> * sysdeps/generic/Makefile (sysdep_headers): Add
> malloc-sysdep.h as a dependency when building malloc.
> * sysdeps/generic/malloc-sysdep.h: New file. Define
> new function check_may_shrink_heap.
> * sysdeps/unix/sysv/linux/Makefile (sysdep_headers): Add
> malloc-sysdep.h as dependency when building malloc.
> * sysdeps/unix/sysv/linux/malloc-sysdep.h: New file. Define
> new function check_may_shrink_heap.
>
I have attached an updated patch after Florian pointed out that I had
leaked a file descriptor. The ChangeLog remains the same.
Regards,
Siddhesh
diff --git a/malloc/arena.c b/malloc/arena.c
index 9e5e332..b7aacc4 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)
@@ -618,13 +621,17 @@ static int
shrink_heap(heap_info *h, long diff)
{
long new_size;
+ static int may_shrink_heap = -1;
+
+ if (__builtin_expect (may_shrink_heap < 0, 0))
+ may_shrink_heap = check_may_shrink_heap ();
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 (may_shrink_heap, 0))
{
if((char *)MMAP((char *)h + new_size, diff, PROT_NONE,
MAP_FIXED) == (char *) MAP_FAILED)
diff --git a/sysdeps/generic/Makefile b/sysdeps/generic/Makefile
index f74109d..247cdd3 100644
--- a/sysdeps/generic/Makefile
+++ b/sysdeps/generic/Makefile
@@ -20,6 +20,10 @@ ifeq ($(subdir),string)
CFLAGS-wordcopy.c += -Wno-uninitialized
endif
+ifeq ($(subdir),malloc)
+sysdep_headers += malloc-sysdep.h
+endif
+
ifeq ($(subdir),elf)
ifeq (yes:yes,$(build-shared):$(unwind-find-fde))
# This is needed to support g++ v2 and v3.
diff --git a/sysdeps/generic/malloc-sysdep.h b/sysdeps/generic/malloc-sysdep.h
new file mode 100644
index 0000000..cbb34d6
--- /dev/null
+++ b/sysdeps/generic/malloc-sysdep.h
@@ -0,0 +1,27 @@
+/* Copyright (C) 2012 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+/* Force an unmap on heap shrink for setuid programs. This is so that the
+ kernel immediately drops the PTE for the mapping. */
+
+#include <unistd.h>
+
+static inline int
+check_may_shrink_heap (void)
+{
+ return __libc_enable_secure;
+}
diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index ddae686..f129f45 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -9,6 +9,7 @@ endif
ifeq ($(subdir),malloc)
CFLAGS-malloc.c += -DMORECORE_CLEARS=2
+sysdep_headers += malloc-sysdep.h
endif
ifeq ($(subdir),socket)
diff --git a/sysdeps/unix/sysv/linux/malloc-sysdep.h b/sysdeps/unix/sysv/linux/malloc-sysdep.h
new file mode 100644
index 0000000..bc0ce35
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/malloc-sysdep.h
@@ -0,0 +1,60 @@
+/* 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/>. */
+
+/* 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. */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+static inline int
+check_may_shrink_heap (void)
+{
+ int fd;
+ int ret = 0;
+
+ if (__libc_enable_secure)
+ return 1;
+
+ fd = open ("/proc/sys/vm/overcommit_memory", O_RDONLY);
+
+ if (fd != -1)
+ {
+ char val = 0;
+ int n = read (fd, &val, 1);
+ if (n > 0)
+ val -= '0';
+
+ if (val == 2)
+ ret = 1;
+
+ close (fd);
+ }
+
+ return ret;
+}