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Date: Wed, 24 Nov 2021 07:50:50 +0100
From: Christoph Hellwig <hch@lst.de>
To: Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH 18/29] fsdax: decouple zeroing from the iomap buffered
 I/O code
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On Tue, Nov 23, 2021 at 01:46:35PM -0800, Dan Williams wrote:
> > +               const struct iomap_ops *ops)
> > +{
> > +       unsigned int blocksize = i_blocksize(inode);
> > +       unsigned int off = pos & (blocksize - 1);
> > +
> > +       /* Block boundary? Nothing to do */
> > +       if (!off)
> > +               return 0;
> 
> It took me a moment to figure out why this was correct. I see it was
> also copied from iomap_truncate_page(). It makes sense for DAX where
> blocksize >= PAGE_SIZE so it's always the case that the amount of
> capacity to zero relative to a page is from @pos to the end of the
> block. Is there something else that protects the blocksize < PAGE_SIZE
> case outside of DAX?
> 
> Nothing to change for this patch, just a question I had while reviewing.

This is a helper for truncate ->setattr, where everything outside the
block is deallocated.  So zeroing is only needed inside the block.