[PATCH v2 hmm 09/11] mm/hmm: Poison hmm_range during unregister

[John Hubbard]

On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg at 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 at mellanox.com>
> Reviewed-by: Jérôme Glisse <jglisse at redhat.com>
> ---
> v2
> - Keep range start/end valid after unregistration (Jerome)
> ---
>  mm/hmm.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 6802de7080d172..c2fecb3ecb11e1 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -937,7 +937,7 @@ void hmm_range_unregister(struct hmm_range *range)
>  	struct hmm *hmm = range->hmm;
>  
>  	/* Sanity check this really should not happen. */

That comment can also be deleted, as it has the same meaning as
the WARN_ON() that you just added.

> -	if (hmm == NULL || range->end <= range->start)
> +	if (WARN_ON(range->end <= range->start))
>  		return;
>  
>  	mutex_lock(&hmm->lock);
> @@ -948,7 +948,10 @@ void hmm_range_unregister(struct hmm_range *range)
>  	range->valid = false;
>  	mmput(hmm->mm);
>  	hmm_put(hmm);
> -	range->hmm = NULL;
> +
> +	/* The range is now invalid, leave it poisoned. */

To be precise, we are poisoning the range's back pointer to it's
owning hmm instance.  Maybe this is clearer:

	/*
	 * The range is now invalid, so poison it's hmm pointer. 
	 * Leave other range-> fields in place, for the caller's use.
	 */

...or something like that?

> +	range->valid = false;
> +	memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
>  }
>  EXPORT_SYMBOL(hmm_range_unregister);
>  
> 

The above are very minor documentation points, so:

    Reviewed-by: John Hubbard <jhubbard at nvidia.com>


thanks,
-- 
John Hubbard
NVIDIA