linux-kernel - Re: [PATCH v2 09/10] fs/xfs: Handle CoW for fsdax write() path

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Date:   Wed, 3 Mar 2021 09:57:48 +0000
From:   "ruansy.fnst@...itsu.com" <ruansy.fnst@...itsu.com>
To:     Christoph Hellwig <hch@....de>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "jack@...e.cz" <jack@...e.cz>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        "david@...morbit.com" <david@...morbit.com>,
        "rgoldwyn@...e.de" <rgoldwyn@...e.de>
Subject: Re: [PATCH v2 09/10] fs/xfs: Handle CoW for fsdax write() path

> 
> On Fri, Feb 26, 2021 at 08:20:29AM +0800, Shiyang Ruan wrote:
> >       error = iomap_zero_range(VFS_I(ip), offset, len, NULL,
> > -                     &xfs_buffered_write_iomap_ops);
> > +               IS_DAX(VFS_I(ip)) ?
> > +               &xfs_dax_write_iomap_ops : &xfs_buffered_write_iomap_ops);
> 
> Please add a xfs_zero_range helper that picks the right iomap_ops
> instead of open coding this in a few places.

OK.  I'll add it.
> 
> > +static int
> > +xfs_dax_write_iomap_end(
> > +     struct inode            *inode,
> > +     loff_t                  pos,
> > +     loff_t                  length,
> > +     ssize_t                 written,
> > +     unsigned int            flags,
> > +     struct iomap            *iomap)
> > +{
> > +     int                     error = 0;
> > +     xfs_inode_t             *ip = XFS_I(inode);
> > +
> > +     if (pos + written > i_size_read(inode)) {
> > +             i_size_write(inode, pos + written);
> > +             error = xfs_setfilesize(ip, pos, written);
> > +     }
> > +     if (xfs_is_cow_inode(ip))
> > +             error = xfs_reflink_end_cow(ip, pos, written);
> > +
> > +     return error;
> 
> What is the advantage of the ioemap_end handler here?  It adds another
> indirect funtion call to the fast path, so if we can avoid it, I'd
> rather do that.

These code were in xfs_file_dax_write().  I moved them into the iomap_end
because the mmaped CoW need this.

I know this is not so good, but I could not find another better way. Do you
have any ideas? 

>
> Also, shouldn't we cancel the COW rather than finishing it when setting
> the file size fails?
> 

I did forget about this part.  Thanks for pointing out.


--
Thanks,
Ruan Shiyang.