linux-kernel - Re: [PATCHv5 04/10] node: Link memory nodes to their compute nodes

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Date:   Wed, 6 Feb 2019 09:12:27 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Jonathan Cameron <jonathan.cameron@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCHv5 04/10] node: Link memory nodes to their compute nodes

On Wed, Feb 06, 2019 at 04:26:35AM -0800, Jonathan Cameron wrote:
> On Thu, 24 Jan 2019 16:07:18 -0700
> Keith Busch <keith.busch@...el.com> wrote:
> > +What:		/sys/devices/system/node/nodeX/accessY/initiators/
> > +Date:		December 2018
> > +Contact:	Keith Busch <keith.busch@...el.com>
> > +Description:
> > +		The directory containing symlinks to memory initiator
> > +		nodes that have class "Y" access to this target node's
> > +		memory. CPUs and other memory initiators in nodes not in
> > +		the list accessing this node's memory may have different
> > +		performance.
> 
> Also seems to contain the characteristics of those accesses.

Right, but not in this patch. I will append the description for access
characterisitics in the patch that adds those attributes.
 
> > + * 	This function will export node relationships linking which memory
> > + * 	initiator nodes can access memory targets at a given ranked access
> > + * 	class.
> > + */
> > +int register_memory_node_under_compute_node(unsigned int mem_nid,
> > +					    unsigned int cpu_nid,
> > +					    unsigned access)
> > +{
> > +	struct node *init_node, *targ_node;
> > +	struct node_access_nodes *initiator, *target;
> > +	int ret;
> > +
> > +	if (!node_online(cpu_nid) || !node_online(mem_nid))
> > +		return -ENODEV;
> 
> What do we do under memory/node hotplug?  More than likely that will
> apply in such systems (it does in mine for starters).
> Clearly to do the full story we would need _HMT support etc but
> we can do the prebaked version by having hmat entries for nodes
> that aren't online yet (like we do for SRAT).
> 
> Perhaps one for a follow up patch set.  However, I'd like an
> pr_info to indicate that the node is listed but not online currently.

Yes, hot plug is planned to follow on to this series.

> > +
> > +	init_node = node_devices[cpu_nid];
> > +	targ_node = node_devices[mem_nid];
> > +	initiator = node_init_node_access(init_node, access);
> > +	target = node_init_node_access(targ_node, access);
> > +	if (!initiator || !target)
> > +		return -ENOMEM;
>
> If one of these fails and the other doesn't + the one that succeeded
> did an init, don't we end up leaking a device here?  I'd expect this
> function to not leave things hanging if it has an error. It should
> unwind anything it has done.  It has been added to the list so
> could be cleaned up later, but I'm not seeing that code. 
> 
> These only get cleaned up when the node is removed.

The intiator-target relationship is many-to-many, so we don't want to
free it just because we couldn't allocate its pairing node. The
exisiting one may still be paired to others we were able to allocate.