Re: [PATCH 24/25] xfs: support returning partial reflink results

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: sandeen@redhat.com, linux-nfs@vger.kernel.org,
	linux-cifs@vger.kernel.org, linux-unionfs@vger.kernel.org,
	linux-xfs@vger.kernel.org, linux-mm@kvack.org,
	linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH 24/25] xfs: support returning partial reflink results
Date: Fri, 12 Oct 2018 09:06:30 -0700	[thread overview]
Message-ID: <20181012160630.GE28243@magnolia> (raw)
In-Reply-To: <20181012012226.GT6311@dastard>

On Fri, Oct 12, 2018 at 12:22:26PM +1100, Dave Chinner wrote:
> On Wed, Oct 10, 2018 at 09:15:19PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Back when the XFS reflink code only supported clone_file_range, we were
> > only able to return zero or negative error codes to userspace.  However,
> > now that copy_file_range (which returns bytes copied) can use XFS'
> > clone_file_range, we have the opportunity to return partial results.
> > For example, if userspace sends a 1GB clone request and we run out of
> > space halfway through, we at least can tell userspace that we completed
> > 512M of that request like a regular write.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/xfs_file.c    |    5 +----
> >  fs/xfs/xfs_reflink.c |   19 ++++++++++++++-----
> >  fs/xfs/xfs_reflink.h |    2 +-
> >  3 files changed, 16 insertions(+), 10 deletions(-)
> > 
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index bc9e94bcb7a3..b2b15b8dc4a1 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -928,14 +928,11 @@ xfs_file_remap_range(
> >  	loff_t		len,
> >  	unsigned int	remap_flags)
> >  {
> > -	int		ret;
> > -
> >  	if (!remap_check_flags(remap_flags, RFR_SAME_DATA))
> >  		return -EINVAL;
> >  
> > -	ret = xfs_reflink_remap_range(file_in, pos_in, file_out, pos_out,
> > +	return xfs_reflink_remap_range(file_in, pos_in, file_out, pos_out,
> >  			len, remap_flags);
> > -	return ret < 0 ? ret : len;
> >  }
> >  
> >  STATIC int
> > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > index e1592e751cc2..12a1fe92454e 100644
> > --- a/fs/xfs/xfs_reflink.c
> > +++ b/fs/xfs/xfs_reflink.c
> > @@ -1123,6 +1123,7 @@ xfs_reflink_remap_blocks(
> >  	struct xfs_inode	*dest,
> >  	xfs_fileoff_t		destoff,
> >  	xfs_filblks_t		len,
> > +	xfs_filblks_t		*remapped,
> >  	xfs_off_t		new_isize)
> >  {
> >  	struct xfs_bmbt_irec	imap;
> > @@ -1130,6 +1131,7 @@ xfs_reflink_remap_blocks(
> >  	int			error = 0;
> >  	xfs_filblks_t		range_len;
> >  
> > +	*remapped = 0;
> >  	/* drange = (destoff, destoff + len); srange = (srcoff, srcoff + len) */
> >  	while (len) {
> >  		uint		lock_mode;
> > @@ -1168,6 +1170,7 @@ xfs_reflink_remap_blocks(
> >  		srcoff += range_len;
> >  		destoff += range_len;
> >  		len -= range_len;
> > +		*remapped += range_len;
> >  	}
> 
> So "remapped" is a block count? Can we call this something like
> remap_len so it's obvious what it is tracking?

Ok.

> > @@ -1424,11 +1427,17 @@ xfs_reflink_remap_range(
> >  
> >  	trace_xfs_reflink_remap_range(src, pos_in, len, dest, pos_out);
> >  
> > +	if (len == 0) {
> > +		ret = 0;
> > +		goto out_unlock;
> > +	}
> > +
> >  	dfsbno = XFS_B_TO_FSBT(mp, pos_out);
> >  	sfsbno = XFS_B_TO_FSBT(mp, pos_in);
> >  	fsblen = XFS_B_TO_FSB(mp, len);
> >  	ret = xfs_reflink_remap_blocks(src, sfsbno, dest, dfsbno, fsblen,
> > -			pos_out + len);
> > +			&remapped, pos_out + len);
> > +	remapped = min_t(int64_t, len, XFS_FSB_TO_B(mp, remapped));
> 
> So remapped is returned as a block count, then immediately converted
> to a byte count? Can we return it as byte count so that we don't
> have this weird unit conversion?

But then we'd have a function whose inputs are in units of blocks but
whose return value is in units of bytes.

Maybe I'll just do this to make it more explicit:

xfs_filblks_t	remapped_blocks = 0;
loff_t		remapped_bytes = 0;

ret = xfs_reflink_remap_blocks(..., &remapped_blocks...);
remapped_bytes = min_t(int64_t, len, XFS_FSB_TO_B(mp, remapped_blocks));

...

return remapped_bytes > 0 ? remapped_bytes : ret;

--D

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com