Re: [PATCH] mm, mempolicy: Check parameters first in kernel_get_mempolicy

[Andrew Morton]

On Sat,  1 Aug 2020 17:08:26 +0800 Wenchao Hao <haowenchao22@xxxxxxxxx> wrote:

> Previous implement called untagged_addr before error check, while
> if the error check failed and return EINVAL, the untagged_addr is
> just useless work.
>
> ...
>
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1632,11 +1632,11 @@ static int kernel_get_mempolicy(int __user *policy,
>  	int uninitialized_var(pval);
>  	nodemask_t nodes;
>  
> -	addr = untagged_addr(addr);
> -
>  	if (nmask != NULL && maxnode < nr_node_ids)
>  		return -EINVAL;
>  
> +	addr = untagged_addr(addr);
> +
>  	err = do_get_mempolicy(&pval, &nodes, addr, flags);
>  
>  	if (err)

Well, the compiler will surely avoid that useless work.  But the code
is better this way.