Re: [PATCH 8/9] xfs: add log item precommit operation

[Dave Chinner <david@fromorbit.com>]

On Wed, Jun 29, 2022 at 02:42:43PM -0700, Darrick J. Wong wrote:
> On Thu, Jun 30, 2022 at 07:34:37AM +1000, Dave Chinner wrote:
> > On Wed, Jun 29, 2022 at 02:16:03PM -0700, Darrick J. Wong wrote:
> > > On Mon, Jun 27, 2022 at 10:43:35AM +1000, Dave Chinner wrote:
> > > > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> > > > index 82cf0189c0db..0acb31093d9f 100644
> > > > --- a/fs/xfs/xfs_trans.c
> > > > +++ b/fs/xfs/xfs_trans.c
> > > > @@ -844,6 +844,90 @@ xfs_trans_committed_bulk(
> > > >  	spin_unlock(&ailp->ail_lock);
> > > >  }
> > > >  
> > > > +/*
> > > > + * Sort transaction items prior to running precommit operations. This will
> > > > + * attempt to order the items such that they will always be locked in the same
> > > > + * order. Items that have no sort function are moved to the end of the list
> > > > + * and so are locked last (XXX: need to check the logic matches the comment).
> > > > + *
> > > > + * This may need refinement as different types of objects add sort functions.
> > > > + *
> > > > + * Function is more complex than it needs to be because we are comparing 64 bit
> > > > + * values and the function only returns 32 bit values.
> > > > + */
> > > > +static int
> > > > +xfs_trans_precommit_sort(
> > > > +	void			*unused_arg,
> > > > +	const struct list_head	*a,
> > > > +	const struct list_head	*b)
> > > > +{
> > > > +	struct xfs_log_item	*lia = container_of(a,
> > > > +					struct xfs_log_item, li_trans);
> > > > +	struct xfs_log_item	*lib = container_of(b,
> > > > +					struct xfs_log_item, li_trans);
> > > > +	int64_t			diff;
> > > > +
> > > > +	/*
> > > > +	 * If both items are non-sortable, leave them alone. If only one is
> > > > +	 * sortable, move the non-sortable item towards the end of the list.
> > > > +	 */
> > > > +	if (!lia->li_ops->iop_sort && !lib->li_ops->iop_sort)
> > > > +		return 0;
> > > > +	if (!lia->li_ops->iop_sort)
> > > > +		return 1;
> > > > +	if (!lib->li_ops->iop_sort)
> > > > +		return -1;
> > > > +
> > > > +	diff = lia->li_ops->iop_sort(lia) - lib->li_ops->iop_sort(lib);
> > > 
> > > I'm kinda surprised the iop_sort method doesn't take both log item
> > > pointers, like most sorting-comparator functions?  But I'll see, maybe
> > > you're doing something clever wrt ordering of log items of differing
> > > types, and hence the ->iop_sort implementations are required to return
> > > some absolute priority or something.
> > 
> > Nope, we have to order item locking based on an unchanging
> > characteristic of the object. log items can come and go, we want to
> > lock items in consistent ascending order, so it has to be based on
> > some kind of physical characteristic, like inode number, block
> > address, etc.
> > 
> > e.g. If all objects are ordered by the physical location, we naturally
> > get a lock order that can be applied sanely across differing object
> > types e.g. AG headers will naturally sort and lock before buffers
> > in the AG itself. e.g. inode cluster buffers for unlinked list
> > manipulations will always get locked after the AGI....
> 
> <nod> So if (say) we were going to add dquots to this scheme, we'd
> probably want to shift all the iop_sort functions to return (say) the
> xfs_daddr_t of the associated item?

No, I don't want to tie it specifically to physical address.
ip->i_ino is not a daddr, but it would order just fine against one.

> (Practically speaking, I don't know that I'd want to tie things down to
> the disk address quite this soon, and since it's all incore code anyway
> I don't think the precise type of the return values matter.)

Indeed, I don't even want to tie this specifically to a 64 bit
value; it's intended that objects will return a sort key, and as
we add more object types we'll have to think harder about the
specific key values we use.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com