On Thu, Nov 08, 2018 at 06:17:03PM +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 > > This change avoids a yet-unresolved crash reported in [2]. > > 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, then it would be easy. 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 distance 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 > [2] https://www.spinics.net/lists/arm-kernel/msg683304.html > > Cc: stable@xxxxxxxxxxxxxxx # 4.7 > Signed-off-by: John Garry <john.garry@xxxxxxxxxx> > Acked-by: Will Deacon <will.deacon@xxxxxxx> > --- Applied, thanks. Rob
