Re: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item

[Hugh Dickins]

On Fri, 6 May 2016, zhouchengming wrote:
> On 2016/5/6 5:07, Andrew Morton wrote:
> > On Thu, 5 May 2016 20:42:56 +0800 Zhou Chengming<zhouchengming1@xxxxxxxxxx>
> > wrote:
> > 
> > > A concurrency issue about KSM in the function scan_get_next_rmap_item.
> > > 
> > > task A (ksmd):				|task B (the mm's task):
> > > 					|
> > > mm = slot->mm;				|
> > > down_read(&mm->mmap_sem);		|
> > > 					|
> > > ...					|
> > > 					|
> > > spin_lock(&ksm_mmlist_lock);		|
> > > 					|
> > > ksm_scan.mm_slot go to the next slot;	|
> > > 					|
> > > spin_unlock(&ksm_mmlist_lock);		|
> > > 					|mmput() ->
> > > 					|	ksm_exit():
> > > 					|
> > > 					|spin_lock(&ksm_mmlist_lock);
> > > 					|if (mm_slot&&  ksm_scan.mm_slot !=
> > > mm_slot) {
> > > 					|	if (!mm_slot->rmap_list) {
> > > 					|		easy_to_free = 1;
> > > 					|		...
> > > 					|
> > > 					|if (easy_to_free) {
> > > 					|	mmdrop(mm);
> > > 					|	...
> > > 					|
> > > 					|So this mm_struct will be freed
> > > successfully.

Good catch, yes.  Note that the mmdrop(mm) shown above is not the one that
frees the mm_struct: the whole address space has to be torn down before
we reach the mmdrop(mm) which actually frees the mm_struct.  But you're
right that there's no serialization against ksmd in that interval, so if
ksmd is rescheduled or interrupted for a long time, yes that mm_struct
might be freed by the time of its up_read() below.

> > > 					|
> > > up_read(&mm->mmap_sem);			|
> > > 
> > > As we can see above, the ksmd thread may access a mm_struct that already
> > > been freed to the kmem_cache.
> > > Suppose a fork will get this mm_struct from the kmem_cache, the ksmd
> > > thread
> > > then call up_read(&mm->mmap_sem), will cause mmap_sem.count to become -1.
> > > I changed the scan_get_next_rmap_item function refered to the khugepaged
> > > scan function.
> > 
> > Thanks.
> > 
> > We need to decide whether this fix should be backported into earlier
> > (-stable) kernels.  Can you tell us how easily this is triggered and
> > share your thoughts on this?

Not easy to trigger at all, I think, and I've never seen it or heard
a report of it; but possible.  It can only happen when there are one or
more VM_MERGEABLE areas in the process, but they're all empty or swapped
out when it exits (the easy_to_free route which presents this problem is
only taken in that !mm_slot->rmap_list case - intended to minimize the
drag on quick processes which exit before ksmd even reaches them).

But if ksmd is preempted for a long time in between its spin_unlock
and its up_read, then yes it can happen.  Fix should go back to
2.6.32, I don't think there's been much change here since it went in.

> > 
> > 
> > .
> > 
> 
> I write a patch that can easily trigger this bug.
> When ksmd go to sleep, if a fork get this mm_struct, BUG_ON
> will be triggered.

Please don't use the patch below to test the final version of your fix
(including latest suggestions from Andrea): mm->owner is updated even
before the final mmput() which calls ksm_exit(), so BUGging on a
change of mm->owner says nothing about how likely it would be to
up_read on a freed mm_struct.

Hugh

> 
> From eedfdd12eb11858f69ff4a4300acad42946ca260 Mon Sep 17 00:00:00 2001
> From: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
> Date: Thu, 5 May 2016 17:49:22 +0800
> Subject: [PATCH] ksm: trigger a bug
> 
> Signed-off-by: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
> ---
>  mm/ksm.c |   17 +++++++++++++++++
>  1 files changed, 17 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index ca6d2a0..676368c 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1519,6 +1519,18 @@ static struct rmap_item *get_next_rmap_item(struct
> mm_slot *mm_slot,
>  	return rmap_item;
>  }
> 
> +static void trigger_a_bug(struct task_struct *p, struct mm_struct *mm)
> +{
> +	/* send KILL sig to the task, hope the mm_struct will be freed */
> +	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
> +	/* sleep for 5s, the mm_struct will be freed and another fork
> +	 * will use this mm_struct
> +	 */
> +	schedule_timeout(msecs_to_jiffies(5000));
> +	/* the mm_struct owned by another task */
> +	BUG_ON(mm->owner != p);
> +}
> +
>  static struct rmap_item *scan_get_next_rmap_item(struct page **page)
>  {
>  	struct mm_struct *mm;
> @@ -1526,6 +1538,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct
> page **page)
>  	struct vm_area_struct *vma;
>  	struct rmap_item *rmap_item;
>  	int nid;
> +	struct task_struct *taskp;
> 
>  	if (list_empty(&ksm_mm_head.mm_list))
>  		return NULL;
> @@ -1636,6 +1649,8 @@ next_mm:
>  	remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
> 
>  	spin_lock(&ksm_mmlist_lock);
> +	/* get the mm's task now in the ksm_mmlist_lock */
> +	taskp = mm->owner;
>  	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
>  						struct mm_slot, mm_list);
>  	if (ksm_scan.address == 0) {
> @@ -1651,6 +1666,7 @@ next_mm:
>  		hash_del(&slot->link);
>  		list_del(&slot->mm_list);
>  		spin_unlock(&ksm_mmlist_lock);
> +		trigger_a_bug(taskp, mm);
> 
>  		free_mm_slot(slot);
>  		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
> @@ -1658,6 +1674,7 @@ next_mm:
>  		mmdrop(mm);
>  	} else {
>  		spin_unlock(&ksm_mmlist_lock);
> +		trigger_a_bug(taskp, mm);
>  		up_read(&mm->mmap_sem);
>  	}
> 
> -- 
> 1.7.7