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

[Aneesh Kumar K V]

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