Re: [RFC PATCH 10/11] mm/hmm: Poison hmm_range during unregister

[Souptick Joarder <jrdr.linux@gmail.com>]

On Thu, May 23, 2019 at 9:05 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> From: Jason Gunthorpe <jgg@mellanox.com>
>
> Trying to misuse a range outside its lifetime is a kernel bug. Use WARN_ON
> and poison bytes to detect this condition.
>
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

Acked-by: Souptick Joarder <jrdr.linux@gmail.com>

> ---
>  mm/hmm.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 6c3b7398672c29..02752d3ef2ed92 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -936,8 +936,7 @@ EXPORT_SYMBOL(hmm_range_register);
>   */
>  void hmm_range_unregister(struct hmm_range *range)
>  {
> -       /* Sanity check this really should not happen. */
> -       if (range->hmm == NULL || range->end <= range->start)
> +       if (WARN_ON(range->end <= range->start))
>                 return;

Does it make any sense to sanity check for range == NULL as well ?
>
>         mutex_lock(&range->hmm->lock);
> @@ -945,9 +944,13 @@ void hmm_range_unregister(struct hmm_range *range)
>         mutex_unlock(&range->hmm->lock);
>
>         /* Drop reference taken by hmm_range_register() */
> -       range->valid = false;
>         hmm_put(range->hmm);
> -       range->hmm = NULL;
> +
> +       /* The range is now invalid, leave it poisoned. */
> +       range->valid = false;
> +       range->start = ULONG_MAX;
> +       range->end = 0;
> +       memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
>  }
>  EXPORT_SYMBOL(hmm_range_unregister);
>
> --
> 2.21.0
>