linux-ext4 - Re: [PATCH 18/29] fsdax: decouple zeroing from the iomap buffered I/O code

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Date:   Wed, 24 Nov 2021 07:50:50 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>, Mike Snitzer <snitzer@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>,
        device-mapper development <dm-devel@...hat.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-erofs@...ts.ozlabs.org,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 18/29] fsdax: decouple zeroing from the iomap buffered
 I/O code

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.