Revert "mm: correctly synchronize rss-counters at exit/exec"

This reverts commit 40af1bbd.

It's horribly and utterly broken for at least the following reasons:

 - calling sync_mm_rss() from mmput() is fundamentally wrong, because
   there's absolutely no reason to believe that the task that does the
   mmput() always does it on its own VM.  Example: fork, ptrace, /proc -
   you name it.

 - calling it *after* having done mmdrop() on it is doubly insane, since
   the mm struct may well be gone now.

 - testing mm against NULL before you call it is insane too, since a
NULL mm there would have caused oopses long before.

.. and those are just the three bugs I found before I decided to give up
looking for me and revert it asap.  I should have caught it before I
even took it, but I trusted Andrew too much.

Cc: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -819,6 +819,7 @@ static int exec_mmap(struct mm_struct *mm)
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;
 	old_mm = current->mm;
+	sync_mm_rss(old_mm);
 	mm_release(tsk, old_mm);
 
 	if (old_mm) {

kernel/exit.c

@@ -423,7 +423,6 @@ void daemonize(const char *name, ...)
 	 * user space pages.  We don't need them, and if we didn't close them
 	 * they would be locked into memory.
 	 */
-	mm_release(current, current->mm);
 	exit_mm(current);
 	/*
 	 * We don't want to get frozen, in case system-wide hibernation
@@ -641,6 +640,7 @@ static void exit_mm(struct task_struct * tsk)
 	struct mm_struct *mm = tsk->mm;
 	struct core_state *core_state;
 
+	mm_release(tsk, mm);
 	if (!mm)
 		return;
 	/*
@@ -960,13 +960,9 @@ void do_exit(long code)
 				preempt_count());
 
 	acct_update_integrals(tsk);
-
-	/* Set exit_code before complete_vfork_done() in mm_release() */
-	tsk->exit_code = code;
-
-	/* Release mm and sync mm's RSS info before statistics gathering */
-	mm_release(tsk, tsk->mm);
-
+	/* sync mm's RSS info before statistics gathering */
+	if (tsk->mm)
+		sync_mm_rss(tsk->mm);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
 		hrtimer_cancel(&tsk->signal->real_timer);
@@ -979,6 +975,7 @@ void do_exit(long code)
 		tty_audit_exit();
 	audit_free(tsk);
 
+	tsk->exit_code = code;
 	taskstats_exit(tsk, group_dead);
 
 	exit_mm(tsk);

kernel/fork.c

@@ -619,14 +619,6 @@ void mmput(struct mm_struct *mm)
 			module_put(mm->binfmt->module);
 		mmdrop(mm);
 	}
-
-	/*
-	 * Final rss-counter synchronization. After this point there must be
-	 * no pagefaults into this mm from the current context.  Otherwise
-	 * mm->rss_stat will be inconsistent.
-	 */
-	if (mm)
-		sync_mm_rss(mm);
 }
 EXPORT_SYMBOL_GPL(mmput);