Re: [RFC PATCH 1/2] xfs: add the ability to join a buffer to a defer_ops

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

On Fri, Dec 01, 2017 at 08:36:19AM -0500, Brian Foster wrote:
> On Thu, Nov 30, 2017 at 09:58:05AM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > In certain cases we need to be able to maintain a buffer lock across a
> > defer_finish call.  Since there could be many (large) transactions
> > committed as a result of a defer_finish, we have to hold the buffer
> > across the roll, then immediately rejoin the buffer and mark it dirty in
> > each transaction to keep the log moving forward.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> 
> Seems about right to me. A couple things..
> 
> >  fs/xfs/libxfs/xfs_defer.c |   37 ++++++++++++++++++++++++++++++++++---
> >  fs/xfs/libxfs/xfs_defer.h |    5 ++++-
> >  2 files changed, 38 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> > index 072ebfe..b5b3414 100644
> > --- a/fs/xfs/libxfs/xfs_defer.c
> > +++ b/fs/xfs/libxfs/xfs_defer.c
> > @@ -249,6 +249,10 @@ xfs_defer_trans_roll(
> >  	for (i = 0; i < XFS_DEFER_OPS_NR_INODES && dop->dop_inodes[i]; i++)
> >  		xfs_trans_log_inode(*tp, dop->dop_inodes[i], XFS_ILOG_CORE);
> >  
> > +	/* Hold the (previously bjoin'd) buffer locked across the roll. */
> > +	for (i = 0; i < XFS_DEFER_OPS_NR_BUFS && dop->dop_bufs[i]; i++)
> > +		xfs_trans_bhold(*tp, dop->dop_bufs[i]);
> > +
> 
> It seems more consistent to dirty the buffer in the tx here and
> bjoin+bhold it in the loop below.

I thought the purpose of calling bhold was to prevent the transaction
commit (in xfs_trans_roll) from unlocking the buffer?  Therefore you'd
bhold it before the _roll and then bjoin/dirty the still-locked buffer
afterwards to attach the buffer as a dirty buffer to the new
transaction.

> >  	trace_xfs_defer_trans_roll((*tp)->t_mountp, dop);
> >  
> >  	/* Roll the transaction. */
> > @@ -264,6 +268,12 @@ xfs_defer_trans_roll(
> >  	for (i = 0; i < XFS_DEFER_OPS_NR_INODES && dop->dop_inodes[i]; i++)
> >  		xfs_trans_ijoin(*tp, dop->dop_inodes[i], 0);
> >  
> > +	/* Rejoin the buffers and dirty them so the log moves forward. */
> > +	for (i = 0; i < XFS_DEFER_OPS_NR_BUFS && dop->dop_bufs[i]; i++) {
> > +		xfs_trans_bjoin(*tp, dop->dop_bufs[i]);
> > +		xfs_trans_dirty_buf(*tp, dop->dop_bufs[i]);
> > +	}
> > +
> >  	return error;
> >  }
> >  
> > @@ -299,6 +309,29 @@ xfs_defer_ijoin(
> >  }
> >  
> >  /*
> > + * Add this buffer to the deferred op.  Each joined buffer is relogged
> > + * each time we roll the transaction.
> > + */
> > +int
> > +xfs_defer_bjoin(
> > +	struct xfs_defer_ops		*dop,
> > +	struct xfs_buf			*bp)
> > +{
> > +	int				i;
> > +
> > +	for (i = 0; i < XFS_DEFER_OPS_NR_BUFS; i++) {
> > +		if (dop->dop_bufs[i] == bp)
> > +			return 0;
> > +		else if (dop->dop_bufs[i] == NULL) {
> > +			dop->dop_bufs[i] = bp;
> > +			return 0;
> > +		}
> > +	}
> > +
> > +	return -EFSCORRUPTED;
> 
> I notice that this looks exactly like xfs_defer_join(), but is
> -EFSCORRUPTED the right error here? It probably doesn't matter that much
> given that if we hit this we've already lost, but I wonder if an error
> that more reflects a programming error as opposed to inconsistent fs
> might be more appropriate..? -EINVAL, -EBUSY?

Yeah, I'm not sure what error code applies to "programmer messed up" :)

Perhaps we should add an ASSERT(0) at the bottom of both functions.

--D

> Brian
> 
> > +}
> > +
> > +/*
> >   * Finish all the pending work.  This involves logging intent items for
> >   * any work items that wandered in since the last transaction roll (if
> >   * one has even happened), rolling the transaction, and finishing the
> > @@ -493,9 +526,7 @@ xfs_defer_init(
> >  	struct xfs_defer_ops		*dop,
> >  	xfs_fsblock_t			*fbp)
> >  {
> > -	dop->dop_committed = false;
> > -	dop->dop_low = false;
> > -	memset(&dop->dop_inodes, 0, sizeof(dop->dop_inodes));
> > +	memset(dop, 0, sizeof(struct xfs_defer_ops));
> >  	*fbp = NULLFSBLOCK;
> >  	INIT_LIST_HEAD(&dop->dop_intake);
> >  	INIT_LIST_HEAD(&dop->dop_pending);
> > diff --git a/fs/xfs/libxfs/xfs_defer.h b/fs/xfs/libxfs/xfs_defer.h
> > index d4f046d..045beac 100644
> > --- a/fs/xfs/libxfs/xfs_defer.h
> > +++ b/fs/xfs/libxfs/xfs_defer.h
> > @@ -59,6 +59,7 @@ enum xfs_defer_ops_type {
> >  };
> >  
> >  #define XFS_DEFER_OPS_NR_INODES	2	/* join up to two inodes */
> > +#define XFS_DEFER_OPS_NR_BUFS	2	/* join up to two buffers */
> >  
> >  struct xfs_defer_ops {
> >  	bool			dop_committed;	/* did any trans commit? */
> > @@ -66,8 +67,9 @@ struct xfs_defer_ops {
> >  	struct list_head	dop_intake;	/* unlogged pending work */
> >  	struct list_head	dop_pending;	/* logged pending work */
> >  
> > -	/* relog these inodes with each roll */
> > +	/* relog these with each roll */
> >  	struct xfs_inode	*dop_inodes[XFS_DEFER_OPS_NR_INODES];
> > +	struct xfs_buf		*dop_bufs[XFS_DEFER_OPS_NR_BUFS];
> >  };
> >  
> >  void xfs_defer_add(struct xfs_defer_ops *dop, enum xfs_defer_ops_type type,
> > @@ -77,6 +79,7 @@ void xfs_defer_cancel(struct xfs_defer_ops *dop);
> >  void xfs_defer_init(struct xfs_defer_ops *dop, xfs_fsblock_t *fbp);
> >  bool xfs_defer_has_unfinished_work(struct xfs_defer_ops *dop);
> >  int xfs_defer_ijoin(struct xfs_defer_ops *dop, struct xfs_inode *ip);
> > +int xfs_defer_bjoin(struct xfs_defer_ops *dop, struct xfs_buf *bp);
> >  
> >  /* Description of a deferred type. */
> >  struct xfs_defer_op_type {
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html