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Date:   Mon, 20 Jan 2020 10:36:46 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com
Subject: Re: [PATCH 1/8] mm/migrate.c: skip node check if done in last round
Message-ID: <20200120093646.GL18451@dhcp22.suse.cz>
References: <20200119030636.11899-1-richardw.yang@linux.intel.com>
 <20200119030636.11899-2-richardw.yang@linux.intel.com>
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On Sun 19-01-20 11:06:29, Wei Yang wrote:
> Before move page to target node, we would check if the node id is valid.
> In case we would try to move pages to the same target node, it is not
> necessary to do the check each time.
> 
> This patch tries to skip the check if the node has been checked.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>  mm/migrate.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 430fdccc733e..ba7cf4fa43a0 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1612,15 +1612,18 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>  			goto out_flush;
>  		addr = (unsigned long)untagged_addr(p);
>  
> -		err = -ENODEV;
> -		if (node < 0 || node >= MAX_NUMNODES)
> -			goto out_flush;
> -		if (!node_state(node, N_MEMORY))
> -			goto out_flush;
> +		/* Check node if it is not checked. */
> +		if (current_node == NUMA_NO_NODE || node != current_node) {
> +			err = -ENODEV;
> +			if (node < 0 || node >= MAX_NUMNODES)
> +				goto out_flush;
> +			if (!node_state(node, N_MEMORY))
> +				goto out_flush;

This makes the code harder to read IMHO. The original code checks the
valid node first and it doesn't conflate that with the node caching
logic which your change does.

>  
> -		err = -EACCES;
> -		if (!node_isset(node, task_nodes))
> -			goto out_flush;
> +			err = -EACCES;
> +			if (!node_isset(node, task_nodes))
> +				goto out_flush;
> +		}
>  
>  		if (current_node == NUMA_NO_NODE) {
>  			current_node = node;
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs