Re: [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter()

[Yang Shi]

On Fri, Aug 14, 2020 at 3:13 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxx>
> Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v4.8+

Acked-by: Yang Shi <shy828301@xxxxxxxxx>

> ---
>
>  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;
>