Re: [PATCH 07/11] xfs: split unaligned DIO write code out

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jan 18, 2021 at 08:35:12PM +0100, Christoph Hellwig wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> The unaligned DIO write path is more convolted than the normal path,
> and we are about to make it more complex. Keep the block aligned
> fast path dio write code trim and simple by splitting out the
> unaligned DIO code from it.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> [hch: rebased, fixed a few minor nits]
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks good,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/xfs_file.c | 168 +++++++++++++++++++++++++---------------------
>  1 file changed, 92 insertions(+), 76 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index a696bd34f71d21..bffd7240cefb7f 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -500,117 +500,133 @@ static const struct iomap_dio_ops xfs_dio_write_ops = {
>  };
>  
>  /*
> - * xfs_file_dio_write - handle direct IO writes
> + * Handle block aligned direct IO writes
>   *
>   * Lock the inode appropriately to prepare for and issue a direct IO write.
> - * By separating it from the buffered write path we remove all the tricky to
> - * follow locking changes and looping.
>   *
>   * If there are cached pages or we're extending the file, we need IOLOCK_EXCL
>   * until we're sure the bytes at the new EOF have been zeroed and/or the cached
>   * pages are flushed out.
> + */
> +static noinline ssize_t
> +xfs_file_dio_write_aligned(
> +	struct xfs_inode	*ip,
> +	struct kiocb		*iocb,
> +	struct iov_iter		*from)
> +{
> +	int			iolock = XFS_IOLOCK_SHARED;
> +	ssize_t			ret;
> +
> +	ret = xfs_ilock_iocb(iocb, iolock);
> +	if (ret)
> +		return ret;
> +	ret = xfs_file_write_checks(iocb, from, &iolock);
> +	if (ret)
> +		goto out_unlock;
> +
> +	/*
> +	 * We don't need to hold the IOLOCK exclusively across the IO, so demote
> +	 * the iolock back to shared if we had to take the exclusive lock in
> +	 * xfs_file_write_checks() for other reasons.
> +	 */
> +	if (iolock == XFS_IOLOCK_EXCL) {
> +		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
> +		iolock = XFS_IOLOCK_SHARED;
> +	}
> +	trace_xfs_file_direct_write(iocb, from);
> +	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> +			   &xfs_dio_write_ops, is_sync_kiocb(iocb));
> +out_unlock:
> +	if (iolock)
> +		xfs_iunlock(ip, iolock);
> +	return ret;
> +}
> +
> +/*
> + * Handle block unaligned direct IO writes
> + *
> + * In most cases direct IO writes will be done holding IOLOCK_SHARED, allowing
> + * them to be done in parallel with reads and other direct IO writes.  However,
> + * if the I/O is not aligned to filesystem blocks, the direct I/O layer may
> + * need to do sub-block zeroing and that requires serialisation against other
> + * direct I/Os to the same block. In this case we need to serialise the
> + * submission of the unaligned I/Os so that we don't get racing block zeroing in
> + * the dio layer.
>   *
> - * In most cases the direct IO writes will be done holding IOLOCK_SHARED
> - * allowing them to be done in parallel with reads and other direct IO writes.
> - * However, if the IO is not aligned to filesystem blocks, the direct IO layer
> - * needs to do sub-block zeroing and that requires serialisation against other
> - * direct IOs to the same block. In this case we need to serialise the
> - * submission of the unaligned IOs so that we don't get racing block zeroing in
> - * the dio layer.  To avoid the problem with aio, we also need to wait for
> + * To provide the same serialisation for AIO, we also need to wait for
>   * outstanding IOs to complete so that unwritten extent conversion is completed
>   * before we try to map the overlapping block. This is currently implemented by
>   * hitting it with a big hammer (i.e. inode_dio_wait()).
>   *
> - * Returns with locks held indicated by @iolock and errors indicated by
> - * negative return values.
> + * This means that unaligned dio writes always block. There is no "nowait" fast
> + * path in this code - if IOCB_NOWAIT is set we simply return -EAGAIN up front
> + * and we don't have to worry about that anymore.
>   */
> -STATIC ssize_t
> -xfs_file_dio_write(
> +static noinline ssize_t
> +xfs_file_dio_write_unaligned(
> +	struct xfs_inode	*ip,
>  	struct kiocb		*iocb,
>  	struct iov_iter		*from)
>  {
> -	struct file		*file = iocb->ki_filp;
> -	struct address_space	*mapping = file->f_mapping;
> -	struct inode		*inode = mapping->host;
> -	struct xfs_inode	*ip = XFS_I(inode);
> -	struct xfs_mount	*mp = ip->i_mount;
> -	ssize_t			ret = 0;
> -	int			unaligned_io = 0;
> -	int			iolock;
> -	size_t			count = iov_iter_count(from);
> -	struct xfs_buftarg      *target = xfs_inode_buftarg(ip);
> +	int			iolock = XFS_IOLOCK_EXCL;
> +	ssize_t			ret;
>  
> -	/* DIO must be aligned to device logical sector size */
> -	if ((iocb->ki_pos | count) & target->bt_logical_sectormask)
> -		return -EINVAL;
> +	/* unaligned dio always waits, bail */
> +	if (iocb->ki_flags & IOCB_NOWAIT)
> +		return -EAGAIN;
> +	xfs_ilock(ip, iolock);
>  
>  	/*
> -	 * Don't take the exclusive iolock here unless the I/O is unaligned to
> -	 * the file system block size.  We don't need to consider the EOF
> -	 * extension case here because xfs_file_write_checks() will relock
> -	 * the inode as necessary for EOF zeroing cases and fill out the new
> -	 * inode size as appropriate.
> +	 * We can't properly handle unaligned direct I/O to reflink files yet,
> +	 * as we can't unshare a partial block.
>  	 */
> -	if ((iocb->ki_pos & mp->m_blockmask) ||
> -	    ((iocb->ki_pos + count) & mp->m_blockmask)) {
> -		unaligned_io = 1;
> -
> -		/*
> -		 * We can't properly handle unaligned direct I/O to reflink
> -		 * files yet, as we can't unshare a partial block.
> -		 */
> -		if (xfs_is_cow_inode(ip)) {
> -			trace_xfs_reflink_bounce_dio_write(iocb, from);
> -			return -ENOTBLK;
> -		}
> -		iolock = XFS_IOLOCK_EXCL;
> -	} else {
> -		iolock = XFS_IOLOCK_SHARED;
> -	}
> -
> -	if (iocb->ki_flags & IOCB_NOWAIT) {
> -		/* unaligned dio always waits, bail */
> -		if (unaligned_io)
> -			return -EAGAIN;
> -		if (!xfs_ilock_nowait(ip, iolock))
> -			return -EAGAIN;
> -	} else {
> -		xfs_ilock(ip, iolock);
> +	if (xfs_is_cow_inode(ip)) {
> +		trace_xfs_reflink_bounce_dio_write(iocb, from);
> +		ret = -ENOTBLK;
> +		goto out_unlock;
>  	}
>  
>  	ret = xfs_file_write_checks(iocb, from, &iolock);
>  	if (ret)
> -		goto out;
> -	count = iov_iter_count(from);
> +		goto out_unlock;
>  
>  	/*
> -	 * If we are doing unaligned IO, we can't allow any other overlapping IO
> -	 * in-flight at the same time or we risk data corruption. Wait for all
> -	 * other IO to drain before we submit. If the IO is aligned, demote the
> -	 * iolock if we had to take the exclusive lock in
> -	 * xfs_file_write_checks() for other reasons.
> +	 * If we are doing unaligned I/O, we can't allow any other overlapping
> +	 * I/O in-flight at the same time or we risk data corruption. Wait for
> +	 * all other I/O to drain before we submit.
>  	 */
> -	if (unaligned_io) {
> -		inode_dio_wait(inode);
> -	} else if (iolock == XFS_IOLOCK_EXCL) {
> -		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
> -		iolock = XFS_IOLOCK_SHARED;
> -	}
> +	inode_dio_wait(VFS_I(ip));
>  
> -	trace_xfs_file_direct_write(iocb, from);
>  	/*
> -	 * If unaligned, this is the only IO in-flight. Wait on it before we
> -	 * release the iolock to prevent subsequent overlapping IO.
> +	 * This must be the only I/O in-flight. Wait on it before we release the
> +	 * iolock to prevent subsequent overlapping I/O.
>  	 */
> +	trace_xfs_file_direct_write(iocb, from);
>  	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> -			   &xfs_dio_write_ops,
> -			   is_sync_kiocb(iocb) || unaligned_io);
> -out:
> +			   &xfs_dio_write_ops, true);
> +out_unlock:
>  	if (iolock)
>  		xfs_iunlock(ip, iolock);
>  	return ret;
>  }
>  
> +static ssize_t
> +xfs_file_dio_write(
> +	struct kiocb		*iocb,
> +	struct iov_iter		*from)
> +{
> +	struct xfs_inode	*ip = XFS_I(file_inode(iocb->ki_filp));
> +	struct xfs_buftarg      *target = xfs_inode_buftarg(ip);
> +	size_t			count = iov_iter_count(from);
> +
> +	/* DIO must be aligned to device logical sector size */
> +	if ((iocb->ki_pos | count) & target->bt_logical_sectormask)
> +		return -EINVAL;
> +	if ((iocb->ki_pos | count) & ip->i_mount->m_blockmask)
> +		return xfs_file_dio_write_unaligned(ip, iocb, from);
> +	return xfs_file_dio_write_aligned(ip, iocb, from);
> +}
> +
>  static noinline ssize_t
>  xfs_file_dax_write(
>  	struct kiocb		*iocb,
> -- 
> 2.29.2
>