Re: [PATCH v9 5/8] fsdax: Add dax_iomap_cow_copy() for dax_iomap_zero

[Darrick J. Wong]

On Thu, Sep 16, 2021 at 04:49:19PM +0800, Shiyang Ruan wrote:
> 
> 
> On 2021/9/16 14:16, Christoph Hellwig wrote:
> > On Wed, Sep 15, 2021 at 06:44:58PM +0800, Shiyang Ruan wrote:
> > > +	rc = dax_direct_access(iomap->dax_dev, pgoff, 1, &kaddr, NULL);
> > > +	if (rc < 0)
> > > +		goto out;
> > > +	memset(kaddr + offset, 0, size);
> > > +	if (srcmap->addr != IOMAP_HOLE && srcmap->addr != iomap->addr) {
> > 
> > Should we also check that ->dax_dev for iomap and srcmap are different
> > first to deal with case of file system with multiple devices?
> 
> I have not thought of this case.  Isn't it possible to CoW between different
> devices?

There's nothing in the iomap API that prevents a filesystem from doing
that, though there are no filesystems today that do such a thing.

That said, if btrfs ever joins the fold (and adds DAX support) then they
could totally COW to a different device.

--D

> 
> 
> --
> Thanks,
> Ruan
> 
> > 
> > Otherwise looks good:
> > 
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > 
> 
>