fork: copy mm's vm usage counters under mmap_sem

If a forking process has a thread calling (un)mmap (silly but still),
the child process may have some of its mm's vm usage counters (total_vm
and friends) screwed up, because currently they are copied from oldmm
w/o holding any locks (memcpy in dup_mm).

This patch moves the counters initialization to dup_mmap() to be called
under oldmm->mmap_sem, which eliminates any possibility of race.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

diff --git a/kernel/fork.c b/kernel/fork.c
index 5a547a5..aff84f8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -374,6 +374,11 @@
 	 */
 	down_write_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
 
+	mm->total_vm = oldmm->total_vm;
+	mm->shared_vm = oldmm->shared_vm;
+	mm->exec_vm = oldmm->exec_vm;
+	mm->stack_vm = oldmm->stack_vm;
+
 	rb_link = &mm->mm_rb.rb_node;
 	rb_parent = NULL;
 	pprev = &mm->mmap;