Re: [PATCH] of, numa: Validate some distance map rules

[Will Deacon <will.deacon@arm.com>]

Hi John,

On Tue, Nov 06, 2018 at 08:39:33PM +0800, John Garry wrote:
> Currently the NUMA distance map parsing does not validate the distance
> table for the distance-matrix rules 1-2 in [1].
> 
> However the arch NUMA code may enforce some of these rules, but not all.
> Such is the case for the arm64 port, which does not enforce the rule that
> the distance between separates nodes cannot equal LOCAL_DISTANCE.
> 
> The patch adds the following rules validation:
> - distance of node to self equals LOCAL_DISTANCE
> - distance of separate nodes > LOCAL_DISTANCE
> 
> A note on dealing with symmetrical distances between nodes:
> 
> Validating symmetrical distances between nodes is difficult. If it were
> mandated in the bindings that every distance must be recorded in the
> table, validating symmetrical distances would be straightforward. However,
> it isn't.
> 
> In addition to this, it is also possible to record [b, a] distance only
> (and not [a, b]). So, when processing the table for [b, a], we cannot
> assert that current distance of [a, b] != [b, a] as invalid, as [a, b]
> distance may not be present in the table and current distance would be
> default at REMOTE_DISTANCE.
> 
> As such, we maintain the policy that we overwrite distance [a, b] = [b, a]
> for b > a. This policy is different to kernel ACPI SLIT validation, which
> allows non-symmetrical distances (ACPI spec SLIT rules allow it). However,
> the debug message is dropped as it may be misleading (for a distance which
> is later overwritten).
> 
> Some final notes on semantics:
> 
> - It is implied that it is the responsibility of the arch NUMA code to
>   reset the NUMA distance map for an error in distance map parsing.
> 
> - It is the responsibility of the FW NUMA topology parsing (whether OF or
>   ACPI) to enforce NUMA distance rules, and not arch NUMA code.
> 
> [1] Documents/devicetree/bindings/numa.txt
> 
> Signed-off-by: John Garry <john.garry@huawei.com>

Is it worth mentioning that the lack of this check was leading to a kernel
crash with a malformed DT entry?

> diff --git a/drivers/of/of_numa.c b/drivers/of/of_numa.c
> index 35c64a4295e0..fe6b13608e51 100644
> --- a/drivers/of/of_numa.c
> +++ b/drivers/of/of_numa.c
> @@ -104,9 +104,14 @@ static int __init of_numa_parse_distance_map_v1(struct device_node *map)
>  		distance = of_read_number(matrix, 1);
>  		matrix++;
>  
> +		if ((nodea == nodeb && distance != LOCAL_DISTANCE) ||
> +		    (nodea != nodeb && distance <= LOCAL_DISTANCE)) {
> +			pr_err("Invalid distance[node%d -> node%d] = %d\n",
> +			       nodea, nodeb, distance);
> +			return -EINVAL;
> +		}
> +
>  		numa_set_distance(nodea, nodeb, distance);
> -		pr_debug("distance[node%d -> node%d] = %d\n",
> -			 nodea, nodeb, distance);

Looks good to me, although I'm not sure which tree this should go through.

Acked-by: Will Deacon <will.deacon@arm.com>

Will