Re: [RFC PATCH v4 4/7] mm/demotion/dax/kmem: Set node's memory tier to MEMORY_TIER_PMEM

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:

> On 6/6/22 3:41 PM, Bharata B Rao wrote:
>> On 6/3/2022 2:34 PM, Aneesh Kumar K V wrote:
>>> On 6/2/22 12:06 PM, Bharata B Rao wrote:
>>>> On 6/1/2022 7:19 PM, Aneesh Kumar K V wrote:
>>>>> On 6/1/22 11:59 AM, Bharata B Rao wrote:
>>>>>> I was experimenting with this patchset and found this behaviour.
>>>>>> Here's what I did:
>>>>>>
>>>>>> Boot a KVM guest with vNVDIMM device which ends up with device_dax
>>>>>> driver by default.
>>>>>>
>>>>>> Use it as RAM by binding it to dax kmem driver. It now appears as
>>>>>> RAM with a new NUMA node that is put to memtier1 (the existing tier
>>>>>> where DRAM already exists)
>>>>>>
>>>>>
>>>>> That should have placed it in memtier2.
>>>>>
>>>>>> I can move it to memtier2 (MEMORY_RANK_PMEM) manually, but isn't
>>>>>> that expected to happen automatically when a node with dax kmem
>>>>>> device comes up?
>>>>>>
>>>>>
>>>>> This can happen if we have added the same NUMA node to memtier1 before dax kmem driver initialized the pmem memory. Can you check before the above node_set_memory_tier_rank() whether the specific NUMA node is already part of any memory tier?
>>>>
>>>> When we reach node_set_memory_tier_rank(), node1 (that has the pmem device)
>>>> is already part of memtier1 whose nodelist shows 0-1.
>>>>
>>>
>>> can you find out which code path added node1 to memtier1?
>> 
>>   node_set_memory_tier_rank+0x63/0x80
>>   migrate_on_reclaim_callback+0x40/0x4d
>>   blocking_notifier_call_chain+0x68/0x90
>>   memory_notify+0x1b/0x20
>>   online_pages+0x257/0x2f0
>>   memory_subsys_online+0x99/0x150
>>   device_online+0x65/0x90
>>   online_memory_block+0x1b/0x20
>>   walk_memory_blocks+0x85/0xc0
>>   ? generic_online_page+0x40/0x40
>>   add_memory_resource+0x1fa/0x2d0
>>   add_memory_driver_managed+0x80/0xc0
>>   dev_dax_kmem_probe+0x1af/0x250
>>   dax_bus_probe+0x6e/0xa0
>> 
>> After this the explicit call to node_set_memory_tier_rank(numa_node, MEMORY_RANK_PMEM)
>> from dev_dax_kmem_probe() finds that the memtier is already set.
>> 
>>> Do you have regular memory also appearing on node1?
>> 
>> No, regular memory is on Node0.
>> 
>
> Thanks for the stack trace. I was getting the kvm setup on my laptop to 
> test this. We should move node_set_mem_tier() early. You had automatic 
> online on memory hotplug
>
> 	/* online pages if requested */
> 	if (mhp_default_online_type != MMOP_OFFLINE)
> 		walk_memory_blocks(start, size, NULL, online_memory_block);
>
>
> which caused memory to be onlined before we could do node_set_mem_tier. 
> That is a bug on my side. Will send you a change after testing .
>
Can you try this change?

diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
index 7a11c387fbbc..905609260dda 100644
--- a/drivers/dax/kmem.c
+++ b/drivers/dax/kmem.c
@@ -94,6 +94,17 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 		goto err_reg_mgid;
 	data->mgid = rc;
 
+	/*
+	 * This get called before the node is brought online. That
+	 * is because depending on the value of mhp_default_online_type
+	 * the kernel will online the memory along with hotplug
+	 * operation. Add the new memory tier before we try to bring
+	 * memory blocks online. Otherwise new node will get added to
+	 * the default memory tier via hotplug callbacks.
+	 */
+#ifdef CONFIG_TIERED_MEMORY
+	node_set_memory_tier(numa_node, MEMORY_TIER_PMEM);
+#endif
 	for (i = 0; i < dev_dax->nr_range; i++) {
 		struct resource *res;
 		struct range range;
@@ -148,9 +159,6 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 
 	dev_set_drvdata(dev, data);
 
-#ifdef CONFIG_TIERED_MEMORY
-	node_set_memory_tier(numa_node, MEMORY_TIER_PMEM);
-#endif
 	return 0;
 
 err_request_mem: