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

Aneesh Kumar K V
2022-06-06T10:23:39Z

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 .

-aneesh