linux-kernel - Re: [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged

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Message-ID: <CAHbLzkpYWxGiXqyaF167dj_qFo1apchryGHv_RG4UKQgY5-jOQ@mail.gmail.com>
Date:   Mon, 17 Aug 2020 13:50:28 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Song Liu <songliubraving@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter()

On Fri, Aug 14, 2020 at 3:13 PM Hugh Dickins <hughd@...gle.com> 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@...glegroups.com>
> Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: stable@...r.kernel.org # v4.8+

Acked-by: Yang Shi <shy828301@...il.com>

> ---
>
>  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;
>