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

[zhouchengming]

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.
> > 					|
> > 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?
>
>
> .

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.

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