From: Hugh Dickins hughd@google.com Subject: 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().

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008141503370.18085@eggly.anvils Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()") Signed-off-by: Hugh Dickins hughd@google.com Reported-by: syzbot syzkaller@googlegroups.com Acked-by: Yang Shi shy828301@gmail.com Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Song Liu songliubraving@fb.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Eric Dumazet edumazet@google.com Cc: stable@vger.kernel.org [4.8+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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

--- a/mm/khugepaged.c~khugepaged-adjust-vm_bug_on_mm-in-__khugepaged_enter +++ a/mm/khugepaged.c @@ -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; _