Andrew Morton wrote:
On Tue, 10 Apr 2012 21:07:32 +0400
Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx> wrote:
mm->rss_stat counters have per-task delta: task->rss_stat, before changing
task->mm pointer kernel must flush this delta with help of sync_mm_rss().
do_exit() already calls sync_mm_rss() to flush rss-counters before commiting
rss-statistics into task->signal->maxrss, taskstats, audit and other stuff.
Unfortunately kernel do this before calling mm_relese(), which can call put_user()
for processing task->clear_child_tid. So at this point we can trigger page-faults
and task->rss_stat becomes non-zero again, as result mm->rss_stat becomes
inconsistent and check_mm() will print something like this:
| BUG: Bad rss-counter state mm:ffff88020813c380 idx:1 val:-1
| BUG: Bad rss-counter state mm:ffff88020813c380 idx:2 val:1
This patch moves sync_mm_rss() into mm_release(), and moves mm_release() out of
do_exit() and calls it earlier. After mm_release() there should be no page-faults.
...
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -751,6 +751,14 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
}
tsk->clear_child_tid = NULL;
}
+
+ /*
+ * Final rss-counter synchronization. After this point must be
+ * no page-faults into this mm from current context, otherwise
+ * mm->rss_stat will be inconsistent.
+ */
+ if (mm)
+ sync_mm_rss(mm);
}
Well that's scary. AFACIT `mm' can indeed be NULL here, when a kernel
thread calls do_exit(). No implementation of deactivate_mm() actually
uses its `mm' arg and I guess that kernel threads never set
tsk->clear_child_tid. Whee.
But it works as designed =)
Do we think we should backport this into -stable kernels? How hard is
it to make that warning come out?
Warning was introduced in v3.3-3720-gc3f0327, so no releases with them.
And I think older kernels can live fine with slightly racy rss counters.
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