Re: [PATCH RFC 07/10] xfs: Add xfs_file_dio_write_atomic()

[John Garry]

On 05/02/2025 19:55, Darrick J. Wong wrote:
> On Tue, Feb 04, 2025 at 12:01:24PM +0000, John Garry wrote:
> > Add xfs_file_dio_write_atomic() for dedicated handling of atomic writes.
> >
> > In case of -EAGAIN being returned from iomap_dio_rw(), reissue the write
> > in CoW-based atomic write mode.
> >
> > In the CoW-based atomic write mode, first unshare blocks so that we don't
> > have a cow fork for the data in the range which we are writing.
> >
> > Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> > ---
> >   fs/xfs/xfs_file.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 51 insertions(+)
> >
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index fd05b66aea3f..12af5cdc3094 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -619,6 +619,55 @@ xfs_file_dio_write_aligned(
> >   	return ret;
> >   }
> >   
> > +static noinline ssize_t
> > +xfs_file_dio_write_atomic(
> > +	struct xfs_inode	*ip,
> > +	struct kiocb		*iocb,
> > +	struct iov_iter		*from)
> > +{
> > +	unsigned int		iolock = XFS_IOLOCK_SHARED;
> > +	bool			use_cow = false;
> > +	unsigned int		dio_flags;
> > +	ssize_t			ret;
> > +
> > +retry:
> > +	ret = xfs_ilock_iocb_for_write(iocb, &iolock);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = xfs_file_write_checks(iocb, from, &iolock);
> > +	if (ret)
> > +		goto out_unlock;
> > +
> > +	if (use_cow) {
> > +		ret = xfs_reflink_unshare(ip, iocb->ki_pos,
> > +			iov_iter_count(from));
>
> Nit: continuation lines should be indented two tabs:
>
> 		ret = xfs_reflink_unshare(ip, iocb->ki_pos,
> 				iov_iter_count(from));

ok

> > +		if (ret)
> > +			goto out_unlock;
> > +	}
> > +
> > +	trace_xfs_file_direct_write(iocb, from);
> > +	if (use_cow)
> > +		dio_flags = IOMAP_DIO_ATOMIC_COW;
> > +	else
> > +		dio_flags = 0;
>
> I also think you could eliminate use_cow by initializing dio_flags to
> zero at the top, OR'ing in IOMAP_DIO_ATOMIC_COW in the retry clause
> below, and using (dio_flags & IOMAP_DIO_ATOMIC_COW) to determine if you
> should call unshare above.

ok, fine, if you think that it is better

> Note: This serializes all the software untorn direct writes.  I think
> a more performant solution would allocate the cow staging blocks ondisk,
> attach them to the directio ioend context, and alter ->iomap_begin and
> the ioend remap to use the attached blocks, but that's a lot more
> surgery.

sure, that does sound like it's quite intrusive. But whatever we do I 
would like to keep the behaviour that racing reads and atomic writes 
mean that a read sees all old or all new data. That is how SCSI and NVMe 
behaves, even though it is not an advertised atomic write feature.

Thanks,
John