Re: [PATCH 1/2] xfs: trylock underlying buffer on dquot flush

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Mar 26, 2020 at 09:17:02AM -0400, Brian Foster wrote:
> A dquot flush currently blocks on the buffer lock for the underlying
> dquot buffer. In turn, this causes xfsaild to block rather than
> continue processing other items in the meantime. Update
> xfs_qm_dqflush() to trylock the buffer, similar to how inode buffers
> are handled, and return -EAGAIN if the lock fails. Fix up any
> callers that don't currently handle the error properly.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

Is xfs_qm_dquot_isolate returning LRU_RETRY an acceptable resolution (as
opposed to, say, LRU_SKIP) for xfs_qm_dqflush returning -EAGAIN?

--D

> ---
>  fs/xfs/xfs_dquot.c      |  6 +++---
>  fs/xfs/xfs_dquot_item.c |  3 ++-
>  fs/xfs/xfs_qm.c         | 14 +++++++++-----
>  3 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
> index 711376ca269f..af2c8e5ceea0 100644
> --- a/fs/xfs/xfs_dquot.c
> +++ b/fs/xfs/xfs_dquot.c
> @@ -1105,8 +1105,8 @@ xfs_qm_dqflush(
>  	 * Get the buffer containing the on-disk dquot
>  	 */
>  	error = xfs_trans_read_buf(mp, NULL, mp->m_ddev_targp, dqp->q_blkno,
> -				   mp->m_quotainfo->qi_dqchunklen, 0, &bp,
> -				   &xfs_dquot_buf_ops);
> +				   mp->m_quotainfo->qi_dqchunklen, XBF_TRYLOCK,
> +				   &bp, &xfs_dquot_buf_ops);
>  	if (error)
>  		goto out_unlock;
>  
> @@ -1177,7 +1177,7 @@ xfs_qm_dqflush(
>  
>  out_unlock:
>  	xfs_dqfunlock(dqp);
> -	return -EIO;
> +	return error;
>  }
>  
>  /*
> diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
> index cf65e2e43c6e..baad1748d0d1 100644
> --- a/fs/xfs/xfs_dquot_item.c
> +++ b/fs/xfs/xfs_dquot_item.c
> @@ -189,7 +189,8 @@ xfs_qm_dquot_logitem_push(
>  		if (!xfs_buf_delwri_queue(bp, buffer_list))
>  			rval = XFS_ITEM_FLUSHING;
>  		xfs_buf_relse(bp);
> -	}
> +	} else if (error == -EAGAIN)
> +		rval = XFS_ITEM_LOCKED;
>  
>  	spin_lock(&lip->li_ailp->ail_lock);
>  out_unlock:
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index de1d2c606c14..68c778d25c48 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -121,12 +121,11 @@ xfs_qm_dqpurge(
>  {
>  	struct xfs_mount	*mp = dqp->q_mount;
>  	struct xfs_quotainfo	*qi = mp->m_quotainfo;
> +	int			error = -EAGAIN;
>  
>  	xfs_dqlock(dqp);
> -	if ((dqp->dq_flags & XFS_DQ_FREEING) || dqp->q_nrefs != 0) {
> -		xfs_dqunlock(dqp);
> -		return -EAGAIN;
> -	}
> +	if ((dqp->dq_flags & XFS_DQ_FREEING) || dqp->q_nrefs != 0)
> +		goto out_unlock;
>  
>  	dqp->dq_flags |= XFS_DQ_FREEING;
>  
> @@ -139,7 +138,6 @@ xfs_qm_dqpurge(
>  	 */
>  	if (XFS_DQ_IS_DIRTY(dqp)) {
>  		struct xfs_buf	*bp = NULL;
> -		int		error;
>  
>  		/*
>  		 * We don't care about getting disk errors here. We need
> @@ -149,6 +147,8 @@ xfs_qm_dqpurge(
>  		if (!error) {
>  			error = xfs_bwrite(bp);
>  			xfs_buf_relse(bp);
> +		} else if (error == -EAGAIN) {
> +			goto out_unlock;
>  		}
>  		xfs_dqflock(dqp);
>  	}
> @@ -174,6 +174,10 @@ xfs_qm_dqpurge(
>  
>  	xfs_qm_dqdestroy(dqp);
>  	return 0;
> +
> +out_unlock:
> +	xfs_dqunlock(dqp);
> +	return error;
>  }
>  
>  /*
> -- 
> 2.21.1
>