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

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On Fri, Mar 27, 2020 at 12:44:40PM -0400, Brian Foster wrote:
> On Fri, Mar 27, 2020 at 08:45:28AM -0700, Darrick J. Wong wrote:
> > 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@xxxxxxxxxx>
> > 
> > Is xfs_qm_dquot_isolate returning LRU_RETRY an acceptable resolution (as
> > opposed to, say, LRU_SKIP) for xfs_qm_dqflush returning -EAGAIN?
> > 
> 
> Hmm.. this is reclaim so I suppose LRU_SKIP would be more appropriate
> than retry (along with more consistent with the other trylock failures
> in that function). Ok with something like the following?
> 
> @@ -461,7 +461,11 @@ xfs_qm_dquot_isolate(
>  		spin_unlock(lru_lock);
>  
>  		error = xfs_qm_dqflush(dqp, &bp);
> -		if (error)
> +		if (error == -EAGAIN) {
> +			xfs_dqunlock(dqp);
> +			spin_lock(lru_lock);
> +			goto out_miss_busy;
> +		} else if (error)
>  			goto out_unlock_dirty;

Then again, is it safe to skip from here once we've cycled the lru_lock?

Brian

>  
>  		xfs_buf_delwri_queue(bp, &isol->buffers);
> 
> Brian
> 
> > --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
> > > 
> > 
> 




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