linux-kernel - Re: [PATCH] mm/mempolicy.c: forbid static or relative flags for local NUMA mode

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Date:   Mon, 19 Sep 2016 13:52:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Piotr Kwapulinski <kwapulinski.piotr@...il.com>
Cc:     akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        vbabka@...e.cz, rientjes@...gle.com, mgorman@...hsingularity.net,
        liangchen.linux@...il.com, nzimmer@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mempolicy.c: forbid static or relative flags for
 local NUMA mode

On Sun 18-09-16 13:29:43, Piotr Kwapulinski wrote:
> The MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES flags are irrelevant
> when setting them for MPOL_LOCAL NUMA memory policy via set_mempolicy.
> Return the "invalid argument" from set_mempolicy whenever
> any of these flags is passed along with MPOL_LOCAL.

man 2 set_mempolicy doesn't list this as invalid option. Maybe this is a
documentation bug but is it possible that somebody will see this as an
unexpected error?

> It is consistent with MPOL_PREFERRED passed with empty nodemask.
> It also slightly shortens the execution time in paths where these flags
> are used e.g. when trying to rebind the NUMA nodes for changes in
> cgroups cpuset mems (mpol_rebind_preferred()) or when just printing
> the mempolicy structure (/proc/PID/numa_maps).

I am not sure I understand this argument. What does this patch actually
fix? If this is about the execution time then why not just bail out
early when MPOL_LOCAL && (MPOL_F_STATIC_NODES || MPOL_F_RELATIVE_NODES)

> Isolated tests done.
> 
> Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@...il.com>
> ---
>  mm/mempolicy.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 2da72a5..27b07d1 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -276,7 +276,9 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
>  				return ERR_PTR(-EINVAL);
>  		}
>  	} else if (mode == MPOL_LOCAL) {
> -		if (!nodes_empty(*nodes))
> +		if (!nodes_empty(*nodes) ||
> +		    (flags & MPOL_F_STATIC_NODES) ||
> +		    (flags & MPOL_F_RELATIVE_NODES))
>  			return ERR_PTR(-EINVAL);
>  		mode = MPOL_PREFERRED;
>  	} else if (nodes_empty(*nodes))
> -- 
> 2.9.2

-- 
Michal Hocko
SUSE Labs