[PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item

[Zhou Chengming]

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.

Signed-off-by: Zhou Chengming <zhouchengming1@xxxxxxxxxx>
---
 mm/ksm.c |    7 ++-----
 1 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 7ee101e..6e4324d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1650,6 +1650,7 @@ next_mm:
 	 * because there were no VM_MERGEABLE vmas with such addresses.
 	 */
 	remove_trailing_rmap_items(slot, ksm_scan.rmap_list);
+	up_read(&mm->mmap_sem);
 
 	spin_lock(&ksm_mmlist_lock);
 	ksm_scan.mm_slot = list_entry(slot->mm_list.next,
@@ -1666,16 +1667,12 @@ next_mm:
 		 */
 		hash_del(&slot->link);
 		list_del(&slot->mm_list);
-		spin_unlock(&ksm_mmlist_lock);
 
 		free_mm_slot(slot);
 		clear_bit(MMF_VM_MERGEABLE, &mm->flags);
-		up_read(&mm->mmap_sem);
 		mmdrop(mm);
-	} else {
-		spin_unlock(&ksm_mmlist_lock);
-		up_read(&mm->mmap_sem);
 	}
+	spin_unlock(&ksm_mmlist_lock);
 
 	/* Repeat until we've completed scanning the whole list */
 	slot = ksm_scan.mm_slot;
-- 
1.7.7