[ 11/48] mm: correctly synchronize rss-counters at exit/exec

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>

commit 4fe7efdbdfb1c7e7a7f31decfd831c0f31d37091 upstream.

do_exit() and exec_mmap() call sync_mm_rss() before mm_release() does
put_user(clear_child_tid) which can update task->rss_stat and thus make
mm->rss_stat inconsistent.  This triggers the "BUG:" printk in check_mm().

Let's fix this bug in the safest way, and optimize/cleanup this later.

Reported-by: Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: sync_mm_rss() still takes a struct task_struct *]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -820,10 +820,10 @@
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;
 	old_mm = current->mm;
-	sync_mm_rss(tsk, old_mm);
 	mm_release(tsk, old_mm);
 
 	if (old_mm) {
+		sync_mm_rss(tsk, old_mm);
 		/*
 		 * Make sure that if there is a core dump in progress
 		 * for the old mm, we get out and die instead of going
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -641,6 +641,7 @@
 	mm_release(tsk, mm);
 	if (!mm)
 		return;
+	sync_mm_rss(tsk, mm);
 	/*
 	 * Serialize with any possible pending coredump.
 	 * We must hold mmap_sem around checking core_state


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