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Shrink heap on linux when overcommit_memory == 2 

Using madvise with MADV_DONTNEED to release memory back to the kernel
is not sufficient to change the commit charge accounted against the
process on Linux.  It is OK however, when overcommit is enabled or is
heuristic.  However, when overcommit is restricted to a percentage of
memory setting the contents of /proc/sys/vm/overcommit_memory as 2, it
makes a difference since memory requests will fail.  Hence, we do what
we do with secure exec binaries, which is to call mmap on the region
to be dropped with MAP_FIXED. This internally unmaps the pages in
question and reduces the amount of memory accounted against the
process.
This commit is contained in:
Siddhesh Poyarekar 2012-09-25 14:10:29 +05:30
parent 2b4f00d1a4
commit 9fab36eb58
4 changed files with 98 additions and 3 deletions
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10
ChangeLog
View File

@ -1,3 +1,13 @@
2012-09-25 Siddhesh Poyarekar <siddhesh@redhat.com>
* 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.
2012-09-25 Siddhesh Poyarekar <siddhesh@redhat.com>
* libio/fileops.c (_IO_new_file_seekoff): Fix typos in

9
malloc/arena.c
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@ -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)

25
sysdeps/generic/malloc-sysdep.h Normal file
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@ -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;
}

57
sysdeps/unix/sysv/linux/malloc-sysdep.h Normal file
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@ -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;
}