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Subject[PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter()
syzbot crashes on the VM_BUG_ON_MM(khugepaged_test_exit(mm), mm) in
__khugepaged_enter(): yes, when one thread is about to dump core, has set
core_state, and is waiting for others, another might do something calling
__khugepaged_enter(), which now crashes because I lumped the core_state
test (known as "mmget_still_valid") into khugepaged_test_exit(). I still
think it's best to lump them together, so just in this exceptional case,
check mm->mm_users directly instead of khugepaged_test_exit().

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v4.8+
---

mm/khugepaged.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- v5.9-rc/mm/khugepaged.c 2020-08-12 19:46:50.867196579 -0700
+++ linux/mm/khugepaged.c 2020-08-14 14:24:32.739457309 -0700
@@ -466,7 +466,7 @@ int __khugepaged_enter(struct mm_struct
return -ENOMEM;

/* __khugepaged_exit() must not run from under us */
- VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
+ VM_BUG_ON_MM(atomic_read(&mm->mm_users) == 0, mm);
if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
free_mm_slot(mm_slot);
return 0;
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