Re: [PATCH v3 4/7] s390,dcssblk,dax: Add dax zero_page_range operation to dcssblk driver

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

On Fri,  7 Feb 2020 15:26:49 -0500
Vivek Goyal <vgoyal@redhat.com> wrote:

> Add dax operation zero_page_range for dcssblk driver.
> 
> CC: linux-s390@vger.kernel.org
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  drivers/s390/block/dcssblk.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c
> index 63502ca537eb..331abab5d066 100644
> --- a/drivers/s390/block/dcssblk.c
> +++ b/drivers/s390/block/dcssblk.c
> @@ -57,11 +57,28 @@ static size_t dcssblk_dax_copy_to_iter(struct dax_device *dax_dev,
>  	return copy_to_iter(addr, bytes, i);
>  }
>  
> +static int dcssblk_dax_zero_page_range(struct dax_device *dax_dev, u64 offset,
> +				       size_t len)
> +{
> +	long rc;
> +	void *kaddr;
> +	pgoff_t pgoff = offset >> PAGE_SHIFT;
> +	unsigned page_offset = offset_in_page(offset);
> +
> +	rc = dax_direct_access(dax_dev, pgoff, 1, &kaddr, NULL);

Why do you pass only 1 page as nr_pages argument for dax_direct_access()?
In some other patch in this series there is a comment that this will
currently only be used for one page, but support for more pages might be
added later. Wouldn't it make sense to rather use something like
PAGE_ALIGN(page_offset + len) >> PAGE_SHIFT instead of 1 here, so that
this won't have to be changed when callers will be ready to use it
with more than one page?

Of course, I guess then we'd also need some check on the return value
from dax_direct_access(), i.e. if the returned available range is
large enough for the requested range.