Re: [PATCH v3] ksm: fix conflict between mmput and scan_get_next_rmap_item

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Please ignore this patch v3. I forgot to change the function
unmerge_and_remove_all_rmap_items(). Patch v4 will be the
final version, I think.. Sorry for my carelessness.

Thanks!

On 2016/5/8 14:56, Zhou Chengming 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 may be freed in the mmput().
					|
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.
From the suggestion of Andrea Arcangeli, unmerge_and_remove_all_rmap_items
has the same SMP race condition, so fix it too. My prev fix in function
scan_get_next_rmap_item will introduce a different SMP race condition,
so just invert the up_read/spin_unlock order as Andrea Arcangeli said.

Signed-off-by: Zhou Chengming<zhouchengming1@xxxxxxxxxx>
Suggested-by: Andrea Arcangeli<aarcange@xxxxxxxxxx>
Reviewed-by: Andrea Arcangeli<aarcange@xxxxxxxxxx>
---
  mm/ksm.c |   16 ++++++++++------
  1 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index ca6d2a0..b6dc387 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -777,6 +777,7 @@ static int unmerge_and_remove_all_rmap_items(void)
  		}

  		remove_trailing_rmap_items(mm_slot,&mm_slot->rmap_list);
+		up_read(&mm->mmap_sem);

  		spin_lock(&ksm_mmlist_lock);
  		ksm_scan.mm_slot = list_entry(mm_slot->mm_list.next,
@@ -784,16 +785,12 @@ static int unmerge_and_remove_all_rmap_items(void)
  		if (ksm_test_exit(mm)) {
  			hash_del(&mm_slot->link);
  			list_del(&mm_slot->mm_list);
-			spin_unlock(&ksm_mmlist_lock);

  			free_mm_slot(mm_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);
  	}

  	/* Clean up stable nodes, but don't worry if some are still busy */
@@ -1657,8 +1654,15 @@ next_mm:
  		up_read(&mm->mmap_sem);
  		mmdrop(mm);
  	} else {
-		spin_unlock(&ksm_mmlist_lock);
  		up_read(&mm->mmap_sem);
+		/*
+		 * up_read(&mm->mmap_sem) first because after
+		 * spin_unlock(&ksm_mmlist_lock) run, the "mm" may
+		 * already have been freed under us by __ksm_exit()
+		 * because the "mm_slot" is still hashed and
+		 * ksm_scan.mm_slot doesn't point to it anymore.
+		 */
+		spin_unlock(&ksm_mmlist_lock);
  	}

  	/* Repeat until we've completed scanning the whole list */


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