Re: [PATCH v2] mm: correctly synchronize rss-counters at exit/exec

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 Apr 2012 21:07:32 +0400
Konstantin Khlebnikov <khlebnikov@openvz.org> 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.


Do we think we should backport this into -stable kernels?  How hard is
it to make that warning come out?

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