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    SubjectRe: [Resend with ACK][PATCH] memory hotplug: fix invalid memory access caused by stale kswapd pointer
    On Wed, Jun 20, 2012 at 5:07 PM, Andrew Morton
    <akpm@linux-foundation.org> wrote:
    > On Wed, 20 Jun 2012 17:21:53 +0800
    > Jiang Liu <jiang.liu@huawei.com> wrote:
    >
    >> Function kswapd_stop() will be called to destroy the kswapd work thread
    >> when all memory of a NUMA node has been offlined. But kswapd_stop() only
    >> terminates the work thread without resetting NODE_DATA(nid)->kswapd to NULL.
    >> The stale pointer will prevent kswapd_run() from creating a new work thread
    >> when adding memory to the memory-less NUMA node again. Eventually the stale
    >> pointer may cause invalid memory access.
    >
    > whoops.
    >
    >>
    >> ...
    >>
    >> --- a/mm/vmscan.c
    >> +++ b/mm/vmscan.c
    >> @@ -2961,8 +2961,10 @@ void kswapd_stop(int nid)
    >>  {
    >>       struct task_struct *kswapd = NODE_DATA(nid)->kswapd;
    >>
    >> -     if (kswapd)
    >> +     if (kswapd) {
    >>               kthread_stop(kswapd);
    >> +             NODE_DATA(nid)->kswapd = NULL;
    >> +     }
    >>  }
    >>
    >>  static int __init kswapd_init(void)
    >
    > OK.
    >
    > This function is full of races (ones which we'll never hit ;)) unless
    > the caller provides locking.  It appears that lock_memory_hotplug() is
    > the locking, so I propose this addition:
    >
    > --- a/mm/vmscan.c~memory-hotplug-fix-invalid-memory-access-caused-by-stale-kswapd-pointer-fix
    > +++ a/mm/vmscan.c
    > @@ -2955,7 +2955,8 @@ int kswapd_run(int nid)
    >  }
    >
    >  /*
    > - * Called by memory hotplug when all memory in a node is offlined.
    > + * Called by memory hotplug when all memory in a node is offlined.  Caller must
    > + * hold lock_memory_hotplug().
    >  */
    >  void kswapd_stop(int nid)
    >  {
    > --- a/include/linux/mmzone.h~memory-hotplug-fix-invalid-memory-access-caused-by-stale-kswapd-pointer-fix
    > +++ a/include/linux/mmzone.h
    > @@ -693,7 +693,7 @@ typedef struct pglist_data {
    >                                             range, including holes */
    >        int node_id;
    >        wait_queue_head_t kswapd_wait;
    > -       struct task_struct *kswapd;
    > +       struct task_struct *kswapd;     /* Protected by lock_memory_hotplug() */

    except
    "system_state == SYSTEM_BOOTING"?
    --
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