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

From: Vivek Goyal <vgoyal@redhat.com>
To: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: linux-fsdevel@vger.kernel.org, linux-nvdimm@lists.01.org,
	hch@infradead.org, dm-devel@redhat.com,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH v3 4/7] s390,dcssblk,dax: Add dax zero_page_range operation to dcssblk driver
Date: Tue, 11 Feb 2020 10:11:14 -0500	[thread overview]
Message-ID: <20200211151114.GA8590@redhat.com> (raw)
In-Reply-To: <20200210215315.27b7e310@thinkpad>

On Mon, Feb 10, 2020 at 09:53:15PM +0100, Gerald Schaefer wrote:
> 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.

I left it at 1 page because that's the current limitation of this
interface and there are no callers which are zeroing across page
boundaries.

I prefer to keep it this way and modify it when we are extending this
interface to allow zeroing across page boundaries. Because even if I add
that logic, I can't test it.

But if you still prefer to change it, I am open to make that change.

Thanks
Vivek
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