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

[Bharata B Rao]

On 5/27/2022 5:55 PM, Aneesh Kumar K.V wrote:
> From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> 
> By default, all nodes are assigned to DEFAULT_MEMORY_TIER which
> is memory tier 1 which is designated for nodes with DRAM, so it
> is not the right tier for dax devices.
> 
> Set dax kmem device node's tier to MEMORY_TIER_PMEM, In future,
> support should be added to distinguish the dax-devices which should
> not be MEMORY_TIER_PMEM and right memory tier should be set for them.
> 
> Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> ---
>  drivers/dax/kmem.c | 4 ++++
>  mm/migrate.c       | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
> index a37622060fff..991782aa2448 100644
> --- a/drivers/dax/kmem.c
> +++ b/drivers/dax/kmem.c
> @@ -11,6 +11,7 @@
>  #include <linux/fs.h>
>  #include <linux/mm.h>
>  #include <linux/mman.h>
> +#include <linux/migrate.h>
>  #include "dax-private.h"
>  #include "bus.h"
>  
> @@ -147,6 +148,9 @@ 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

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)

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?

Regards,
Bharata.