Re: [RFC v5 PATCH 7/9] xfs: buffer relogging support prototype

[Dave Chinner <david@fromorbit.com>]

On Mon, Mar 02, 2020 at 02:00:34PM -0500, Brian Foster wrote:
> On Mon, Mar 02, 2020 at 06:47:28PM +1100, Dave Chinner wrote:
> > On Thu, Feb 27, 2020 at 08:43:19AM -0500, Brian Foster wrote:
> > > Add a quick and dirty implementation of buffer relogging support.
> > > There is currently no use case for buffer relogging. This is for
> > > experimental use only and serves as an example to demonstrate the
> > > ability to relog arbitrary items in the future, if necessary.
> > > 
> > > Add a hook to enable relogging a buffer in a transaction, update the
> > > buffer log item handlers to support relogged BLIs and update the
> > > relog handler to join the relogged buffer to the relog transaction.
> > > 
> > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > .....
> > >  /*
> > > @@ -187,9 +188,21 @@ xfs_ail_relog(
> > >  			xfs_log_ticket_put(ailp->ail_relog_tic);
> > >  		spin_unlock(&ailp->ail_lock);
> > >  
> > > -		xfs_trans_add_item(tp, lip);
> > > -		set_bit(XFS_LI_DIRTY, &lip->li_flags);
> > > -		tp->t_flags |= XFS_TRANS_DIRTY;
> > > +		/*
> > > +		 * TODO: Ideally, relog transaction management would be pushed
> > > +		 * down into the ->iop_push() callbacks rather than playing
> > > +		 * games with ->li_trans and looking at log item types here.
> > > +		 */
> > > +		if (lip->li_type == XFS_LI_BUF) {
> > > +			struct xfs_buf_log_item	*bli = (struct xfs_buf_log_item *) lip;
> > > +			xfs_buf_hold(bli->bli_buf);
> > 
> > What is this for? The bli already has a reference to the buffer.
> > 
> 
> The buffer reference is for the transaction. It is analogous to the
> reference acquired in xfs_buf_find() via xfs_trans_[get|read]_buf(), for
> example.

Ah. Comment please :P

> > > +			xfs_trans_bjoin(tp, bli->bli_buf);
> > > +			xfs_trans_dirty_buf(tp, bli->bli_buf);
> > > +		} else {
> > > +			xfs_trans_add_item(tp, lip);
> > > +			set_bit(XFS_LI_DIRTY, &lip->li_flags);
> > > +			tp->t_flags |= XFS_TRANS_DIRTY;
> > > +		}
> > 
> > Really, this should be a xfs_item_ops callout. i.e.
> > 
> > 		lip->li_ops->iop_relog(lip);
> > 
> 
> Yeah, I've already done pretty much this in my local tree. The callback
> also takes the transaction because that's the code that knows how to add
> a particular type of item to a transaction. I didn't require a callback
> for the else case above where no special handling is required
> (quotaoff), so the callback is optional, but I'm not opposed to
> reworking things such that ->iop_relog() is always required if that is
> preferred.

I think I'd prefer to keep things simple right now. Making it an
unconditional callout keeps this code simple, and if there's a
common implementation, add a generic function for it that the items
use.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com