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

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Date:   Wed, 7 Nov 2018 09:55:52 -0600
From:   Rob Herring <robh@...nel.org>
To:     Will Deacon <will.deacon@....com>,
        John Garry <john.garry@...wei.com>
Cc:     frowand.list@...il.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        anshuman.khandual@....com, peterz@...radead.org
Subject: Re: [PATCH] of, numa: Validate some distance map rules

On Wed, Nov 07, 2018 at 03:44:31PM +0000, Will Deacon wrote:
> 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@...wei.com>
> 
> Is it worth mentioning that the lack of this check was leading to a kernel
> crash with a malformed DT entry?

So should be marked for stable too?

> 
> > 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@....com>

I'll take it. Please resend with the comment Will asked for.

Rob