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.
>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 <zhouchengmi...@huawei.com>
Suggested-by: Andrea Arcangeli <aarca...@redhat.com>
---
mm/ksm.c | 17 ++++++++++-------
1 files changed, 10 insertions(+), 7 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index ca6d2a0..d87bafc 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 */
@@ -1650,16 +1647,22 @@ 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);
}
+ /*
+ * 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 */
slot = ksm_scan.mm_slot;
--
1.7.7
