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Subject[PATCH v4] ksm: fix conflict between mmput and scan_get_next_rmap_item
Date
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@huawei.com>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
---
mm/ksm.c | 15 ++++++++++-----
1 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index ca6d2a0..1d4c0e8 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,
@@ -788,12 +789,9 @@ static int unmerge_and_remove_all_rmap_items(void)

free_mm_slot(mm_slot);
clear_bit(MMF_VM_MERGEABLE, &mm->flags);
- up_read(&mm->mmap_sem);
mmdrop(mm);
- } else {
+ } else
spin_unlock(&ksm_mmlist_lock);
- up_read(&mm->mmap_sem);
- }
}

/* Clean up stable nodes, but don't worry if some are still busy */
@@ -1657,8 +1655,15 @@ 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 */
--
1.7.7
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