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

Darrick J. Wong
2025-02-05T19:55:41Z

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@oracle.com>
> ---
> 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));

> + 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.

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.

--D

> +
> + ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> + &xfs_dio_write_ops, dio_flags, NULL, 0);
> +
> + if (ret == -EAGAIN && !(iocb->ki_flags & IOCB_NOWAIT) && !use_cow) {
> + xfs_iunlock(ip, iolock);
> + iolock = XFS_IOLOCK_EXCL;
> + use_cow = true;
> + goto retry;
> + }
> +
> +out_unlock:
> + if (iolock)
> + xfs_iunlock(ip, iolock);
> + return ret;
> +}
> +
> /*
> * Handle block unaligned direct I/O writes
> *
> @@ -723,6 +772,8 @@ xfs_file_dio_write(
> return -EINVAL;
> if ((iocb->ki_pos | count) & ip->i_mount->m_blockmask)
> return xfs_file_dio_write_unaligned(ip, iocb, from);
> + if (iocb->ki_flags & IOCB_ATOMIC)
> + return xfs_file_dio_write_atomic(ip, iocb, from);
> return xfs_file_dio_write_aligned(ip, iocb, from);
> }
>
> --
> 2.31.1
>
>