linux-kernel - Re: [PATCH v9 5/8] fsdax: Add dax_iomap_cow_copy() for dax_iomap

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Message-ID: <20210916225544.GF34830@magnolia>
Date:   Thu, 16 Sep 2021 15:55:44 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        dan.j.williams@...el.com, david@...morbit.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        nvdimm@...ts.linux.dev, rgoldwyn@...e.de, viro@...iv.linux.org.uk,
        willy@...radead.org, Ritesh Harjani <riteshh@...ux.ibm.com>
Subject: Re: [PATCH v9 5/8] fsdax: Add dax_iomap_cow_copy() for dax_iomap_zero

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@....de>
> > 
> 
>