Re: [PATCH 1/5] dax, pmem: Add a dax operation zero_page_range

[Christoph Hellwig <hch@infradead.org>]

> +	/*
> +	 * There are no users as of now. Once users are there, fix dm code
> +	 * to be able to split a long range across targets.
> +	 */

This comment confused me.  I think this wants to say something like:

	/*
	 * There are now callers that want to zero across a page boundary as of
	 * now.  Once there are users this check can be removed after the
	 * device mapper code has been updated to split ranges across targets.
	 */

> +static int pmem_dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> +				    unsigned int offset, size_t len)
> +{
> +	int rc = 0;
> +	phys_addr_t phys_pos = pgoff * PAGE_SIZE + offset;

Any reason not to pass a phys_addr_t in the calling convention for the
method and maybe also for dax_zero_page_range itself?

> +	sector_start = ALIGN(phys_pos, 512)/512;
> +	sector_end = ALIGN_DOWN(phys_pos + bytes, 512)/512;

Missing whitespaces.  Also this could use DIV_ROUND_UP and
DIV_ROUND_DOWN.

> +	if (sector_end > sector_start)
> +		nr_sectors = sector_end - sector_start;
> +
> +	if (nr_sectors &&
> +	    unlikely(is_bad_pmem(&pmem->bb, sector_start,
> +				 nr_sectors * 512)))
> +		bad_pmem = true;

How could nr_sectors be zero?

> +	write_pmem(pmem_addr, page, 0, bytes);
> +	if (unlikely(bad_pmem)) {
> +		/*
> +		 * Pass block aligned offset and length. That seems
> +		 * to work as of now. Other finer grained alignment
> +		 * cases can be addressed later if need be.
> +		 */
> +		rc = pmem_clear_poison(pmem, ALIGN(pmem_off, 512),
> +				       nr_sectors * 512);
> +		write_pmem(pmem_addr, page, 0, bytes);
> +	}

This code largerly duplicates the write side of pmem_do_bvec.  I
think it might make sense to split pmem_do_bvec into a read and a write
side as a prep patch, and then reuse the write side here.

> +int generic_dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
> +				 unsigned int offset, size_t len);

This should probably go into a separare are of the header and have
comment about being a section for generic helpers for drivers.