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