RE: [PATCH v5 2/2] dax/kmem: allow kmem to add memory with memmap_on_memory

[Dan Williams]

Vishal Verma wrote:
> Large amounts of memory managed by the kmem driver may come in via CXL,
> and it is often desirable to have the memmap for this memory on the new
> memory itself.
> 
> Enroll kmem-managed memory for memmap_on_memory semantics if the dax
> region originates via CXL. For non-CXL dax regions, retain the existing
> default behavior of hot adding without memmap_on_memory semantics.
> 
> Add a sysfs override under the dax device to control this behavior and
> override either default.
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> ---
>  drivers/dax/bus.h         |  1 +
>  drivers/dax/dax-private.h |  1 +
>  drivers/dax/bus.c         | 38 ++++++++++++++++++++++++++++++++++++++
>  drivers/dax/cxl.c         |  1 +
>  drivers/dax/hmem/hmem.c   |  1 +
>  drivers/dax/kmem.c        |  8 +++++++-
>  drivers/dax/pmem.c        |  1 +
>  7 files changed, 50 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dax/bus.h b/drivers/dax/bus.h
> index 1ccd23360124..cbbf64443098 100644
> --- a/drivers/dax/bus.h
> +++ b/drivers/dax/bus.h
> @@ -23,6 +23,7 @@ struct dev_dax_data {
>  	struct dev_pagemap *pgmap;
>  	resource_size_t size;
>  	int id;
> +	bool memmap_on_memory;
>  };
>  
>  struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data);
> diff --git a/drivers/dax/dax-private.h b/drivers/dax/dax-private.h
> index 27cf2daaaa79..446617b73aea 100644
> --- a/drivers/dax/dax-private.h
> +++ b/drivers/dax/dax-private.h
> @@ -70,6 +70,7 @@ struct dev_dax {
>  	struct ida ida;
>  	struct device dev;
>  	struct dev_pagemap *pgmap;
> +	bool memmap_on_memory;
>  	int nr_range;
>  	struct dev_dax_range {
>  		unsigned long pgoff;
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index 0ee96e6fc426..43be95a231c9 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -367,6 +367,7 @@ static ssize_t create_store(struct device *dev, struct device_attribute *attr,
>  			.dax_region = dax_region,
>  			.size = 0,
>  			.id = -1,
> +			.memmap_on_memory = false,
>  		};
>  		struct dev_dax *dev_dax = devm_create_dev_dax(&data);
>  
> @@ -1269,6 +1270,40 @@ static ssize_t numa_node_show(struct device *dev,
>  }
>  static DEVICE_ATTR_RO(numa_node);
>  
> +static ssize_t memmap_on_memory_show(struct device *dev,
> +				     struct device_attribute *attr, char *buf)
> +{
> +	struct dev_dax *dev_dax = to_dev_dax(dev);
> +
> +	return sprintf(buf, "%d\n", dev_dax->memmap_on_memory);
> +}
> +
> +static ssize_t memmap_on_memory_store(struct device *dev,
> +				      struct device_attribute *attr,
> +				      const char *buf, size_t len)
> +{
> +	struct dev_dax *dev_dax = to_dev_dax(dev);
> +	struct dax_region *dax_region = dev_dax->region;
> +	ssize_t rc;
> +	bool val;
> +
> +	rc = kstrtobool(buf, &val);
> +	if (rc)
> +		return rc;

Perhaps:

if (dev_dax->memmap_on_memory == val)
	return len;

...and skip the check below when it is going to be a nop

> +
> +	device_lock(dax_region->dev);
> +	if (!dax_region->dev->driver) {

Is the polarity backwards here? I.e. if the device is already attached to
the kmem driver it is too late to modify memmap_on_memory policy.

> +		device_unlock(dax_region->dev);
> +		return -ENXIO;

I would expect -EBUSY since disabling the device allows the property to be
set and -ENXIO implies a more permanent error.

> +	}
> +
> +	dev_dax->memmap_on_memory = val;
> +
> +	device_unlock(dax_region->dev);
> +	return len;
> +}
> +static DEVICE_ATTR_RW(memmap_on_memory);

This new attribute needs a new Documentation/ABI/ entry... in fact all of
these attributes need Documentation/ entries. I can help with that base
document to get things started.

Perhaps split this sysfs ABI into its own patch and, depending on how fast
we can pull the Documentation together, start with the
region-driver-conveyed approach in the meantime.