Re: [PATCH v3 3/4] dm: add infrastructure for DAX support

["Kani, Toshimitsu" <toshi.kani-ZPxbGqLxI0U@public.gmane.org>]

On Thu, 2016-06-23 at 17:05 -0400, Mike Snitzer wrote:
 :
> +static int device_supports_dax(struct dm_target *ti, struct dm_dev *dev,
> +			       sector_t start, sector_t len, void *data)
> +{
> +	struct request_queue *q = bdev_get_queue(dev->bdev);
> +
> +	return q && blk_queue_dax(q);
> +}
> +
> +static bool dm_table_supports_dax(struct dm_table *t)
> +{
> +	struct dm_target *ti;
> +	unsigned i = 0;
> +
> +	/* Ensure that all targets support DAX. */
> +	while (i < dm_table_get_num_targets(t)) {
> +		ti = dm_table_get_target(t, i++);
> +
> +		if (!ti->type->direct_access)
> +			return false;
> +
> +		if (!ti->type->iterate_devices ||
> +		    !ti->type->iterate_devices(ti, device_supports_dax,
> NULL))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +

Hi Mike,

Thanks for the update.  I have a question about the above change.  Targets may
have their own parameters.  For instance, dm-stripe has 'chunk_size', which is
checked in stripe_ctr().  DAX adds additional restriction that chunk_size
needs to be aligned by page size.  So, I think we need to keep target
responsible to verify if DAX can be supported.  What do you think?

Thanks,
-Toshi
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