Re: [PATCH] xfs: abort unaligned nowait directio early

[Brian Foster <bfoster@redhat.com>]

On Wed, Apr 17, 2019 at 07:40:06AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Dave Chinner noticed that xfs_file_dio_aio_write returns EAGAIN without
> dropping the IOLOCK when its deciding not to wait, which means that we
> leak the IOLOCK there.  Since we now make unaligned directio always
> wait, we have the opportunity to bail out before trying to take the
> lock, which should reduce the overhead of this never-gonna-work case
> considerably while also solving the dropped lock problem.
> 
> Reported-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_file.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index cdcc75735521..3a45e3b0efa7 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -517,6 +517,9 @@ xfs_file_dio_aio_write(
>  	}
>  
>  	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 {
> @@ -536,9 +539,6 @@ xfs_file_dio_aio_write(
>  	 * xfs_file_aio_write_checks() for other reasons.
>  	 */
>  	if (unaligned_io) {
> -		/* unaligned dio always waits, bail */
> -		if (iocb->ki_flags & IOCB_NOWAIT)
> -			return -EAGAIN;
>  		inode_dio_wait(inode);
>  	} else if (iolock == XFS_IOLOCK_EXCL) {
>  		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);