Re: [RFC v5 PATCH 3/9] xfs: automatic relogging reservation management

[Allison Collins <allison.henderson@oracle.com>]

On 2/27/20 6:43 AM, Brian Foster wrote:
> Automatic item relogging will occur from xfsaild context. xfsaild
> cannot acquire log reservation itself because it is also responsible
> for writeback and thus making used log reservation available again.
> Since there is no guarantee log reservation is available by the time
> a relogged item reaches the AIL, this is prone to deadlock.
> 
> To guarantee log reservation for automatic relogging, implement a
> reservation management scheme where a transaction that is capable of
> enabling relogging of an item must contribute the necessary
> reservation to the relog mechanism up front. Use reference counting
> to associate the lifetime of pending relog reservation to the
> lifetime of in-core log items with relogging enabled.
> 
> The basic log reservation sequence for a relog enabled transaction
> is as follows:
> 
> - A transaction that uses relogging specifies XFS_TRANS_RELOG at
>    allocation time.
> - Once initialized, RELOG transactions check for the existence of
>    the global relog log ticket. If it exists, grab a reference and
>    return. If not, allocate an empty ticket and install into the relog
>    subsystem. Seed the relog ticket from reservation of the current
>    transaction. Roll the current transaction to replenish its
>    reservation and return to the caller.
> - The transaction is used as normal. If an item is relogged in the
>    transaction, that item acquires a reference on the global relog
>    ticket currently held open by the transaction. The item's reference
>    persists until relogging is disabled on the item.
> - The RELOG transaction commits and releases its reference to the
>    global relog ticket. The global relog ticket is released once its
>    reference count drops to zero.
> 
> This provides a central relog log ticket that guarantees reservation
> availability for relogged items, avoids log reservation deadlocks
> and is allocated and released on demand.
> 
Ok, I followed it through and didn't see any obvious errors
Reviewed-by: Allison Collins <allison.henderson@oracle.com>

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>   fs/xfs/libxfs/xfs_shared.h |  1 +
>   fs/xfs/xfs_trans.c         | 37 +++++++++++++---
>   fs/xfs/xfs_trans.h         |  3 ++
>   fs/xfs/xfs_trans_ail.c     | 89 ++++++++++++++++++++++++++++++++++++++
>   fs/xfs/xfs_trans_priv.h    |  1 +
>   5 files changed, 126 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_shared.h b/fs/xfs/libxfs/xfs_shared.h
> index c45acbd3add9..0a10ca0853ab 100644
> --- a/fs/xfs/libxfs/xfs_shared.h
> +++ b/fs/xfs/libxfs/xfs_shared.h
> @@ -77,6 +77,7 @@ void	xfs_log_get_max_trans_res(struct xfs_mount *mp,
>    * made then this algorithm will eventually find all the space it needs.
>    */
>   #define XFS_TRANS_LOWMODE	0x100	/* allocate in low space mode */
> +#define XFS_TRANS_RELOG		0x200	/* enable automatic relogging */
>   
>   /*
>    * Field values for xfs_trans_mod_sb.
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 3b208f9a865c..8ac05ed8deda 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -107,9 +107,14 @@ xfs_trans_dup(
>   
>   	ntp->t_flags = XFS_TRANS_PERM_LOG_RES |
>   		       (tp->t_flags & XFS_TRANS_RESERVE) |
> -		       (tp->t_flags & XFS_TRANS_NO_WRITECOUNT);
> -	/* We gave our writer reference to the new transaction */
> +		       (tp->t_flags & XFS_TRANS_NO_WRITECOUNT) |
> +		       (tp->t_flags & XFS_TRANS_RELOG);
> +	/*
> +	 * The writer reference and relog reference transfer to the new
> +	 * transaction.
> +	 */
>   	tp->t_flags |= XFS_TRANS_NO_WRITECOUNT;
> +	tp->t_flags &= ~XFS_TRANS_RELOG;
>   	ntp->t_ticket = xfs_log_ticket_get(tp->t_ticket);
>   
>   	ASSERT(tp->t_blk_res >= tp->t_blk_res_used);
> @@ -284,15 +289,25 @@ xfs_trans_alloc(
>   	tp->t_firstblock = NULLFSBLOCK;
>   
>   	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
> -	if (error) {
> -		xfs_trans_cancel(tp);
> -		return error;
> +	if (error)
> +		goto error;
> +
> +	if (flags & XFS_TRANS_RELOG) {
> +		error = xfs_trans_ail_relog_reserve(&tp);
> +		if (error)
> +			goto error;
>   	}
>   
>   	trace_xfs_trans_alloc(tp, _RET_IP_);
>   
>   	*tpp = tp;
>   	return 0;
> +
> +error:
> +	/* clear relog flag if we haven't acquired a ref */
> +	tp->t_flags &= ~XFS_TRANS_RELOG;
> +	xfs_trans_cancel(tp);
> +	return error;
>   }
>   
>   /*
> @@ -973,6 +988,10 @@ __xfs_trans_commit(
>   
>   	xfs_log_commit_cil(mp, tp, &commit_lsn, regrant);
>   
> +	/* release the relog ticket reference if this transaction holds one */
> +	if (tp->t_flags & XFS_TRANS_RELOG)
> +		xfs_trans_ail_relog_put(mp);
> +
>   	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
>   	xfs_trans_free(tp);
>   
> @@ -1004,6 +1023,10 @@ __xfs_trans_commit(
>   			error = -EIO;
>   		tp->t_ticket = NULL;
>   	}
> +	/* release the relog ticket reference if this transaction holds one */
> +	/* XXX: handle RELOG items on transaction abort */
> +	if (tp->t_flags & XFS_TRANS_RELOG)
> +		xfs_trans_ail_relog_put(mp);
>   	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
>   	xfs_trans_free_items(tp, !!error);
>   	xfs_trans_free(tp);
> @@ -1064,6 +1087,10 @@ xfs_trans_cancel(
>   		tp->t_ticket = NULL;
>   	}
>   
> +	/* release the relog ticket reference if this transaction holds one */
> +	if (tp->t_flags & XFS_TRANS_RELOG)
> +		xfs_trans_ail_relog_put(mp);
> +
>   	/* mark this thread as no longer being in a transaction */
>   	current_restore_flags_nested(&tp->t_pflags, PF_MEMALLOC_NOFS);
>   
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 752c7fef9de7..a032989943bd 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -236,6 +236,9 @@ int		xfs_trans_roll_inode(struct xfs_trans **, struct xfs_inode *);
>   void		xfs_trans_cancel(xfs_trans_t *);
>   int		xfs_trans_ail_init(struct xfs_mount *);
>   void		xfs_trans_ail_destroy(struct xfs_mount *);
> +int		xfs_trans_ail_relog_reserve(struct xfs_trans **);
> +bool		xfs_trans_ail_relog_get(struct xfs_mount *);
> +int		xfs_trans_ail_relog_put(struct xfs_mount *);
>   
>   void		xfs_trans_buf_set_type(struct xfs_trans *, struct xfs_buf *,
>   				       enum xfs_blft);
> diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> index 00cc5b8734be..a3fb64275baa 100644
> --- a/fs/xfs/xfs_trans_ail.c
> +++ b/fs/xfs/xfs_trans_ail.c
> @@ -17,6 +17,7 @@
>   #include "xfs_errortag.h"
>   #include "xfs_error.h"
>   #include "xfs_log.h"
> +#include "xfs_log_priv.h"
>   
>   #ifdef DEBUG
>   /*
> @@ -818,6 +819,93 @@ xfs_trans_ail_delete(
>   		xfs_log_space_wake(ailp->ail_mount);
>   }
>   
> +bool
> +xfs_trans_ail_relog_get(
> +	struct xfs_mount	*mp)
> +{
> +	struct xfs_ail		*ailp = mp->m_ail;
> +	bool			ret = false;
> +
> +	spin_lock(&ailp->ail_lock);
> +	if (ailp->ail_relog_tic) {
> +		xfs_log_ticket_get(ailp->ail_relog_tic);
> +		ret = true;
> +	}
> +	spin_unlock(&ailp->ail_lock);
> +	return ret;
> +}
> +
> +/*
> + * Reserve log space for the automatic relogging ->tr_relog ticket. This
> + * requires a clean, permanent transaction from the caller. Pull reservation
> + * for the relog ticket and roll the caller's transaction back to its fully
> + * reserved state. If the AIL relog ticket is already initialized, grab a
> + * reference and return.
> + */
> +int
> +xfs_trans_ail_relog_reserve(
> +	struct xfs_trans	**tpp)
> +{
> +	struct xfs_trans	*tp = *tpp;
> +	struct xfs_mount	*mp = tp->t_mountp;
> +	struct xfs_ail		*ailp = mp->m_ail;
> +	struct xlog_ticket	*tic;
> +	uint32_t		logres = M_RES(mp)->tr_relog.tr_logres;
> +
> +	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
> +	ASSERT(!(tp->t_flags & XFS_TRANS_DIRTY));
> +
> +	if (xfs_trans_ail_relog_get(mp))
> +		return 0;
> +
> +	/* no active ticket, fall into slow path to allocate one.. */
> +	tic = xlog_ticket_alloc(mp->m_log, logres, 1, XFS_TRANSACTION, true, 0);
> +	if (!tic)
> +		return -ENOMEM;
> +	ASSERT(tp->t_ticket->t_curr_res >= tic->t_curr_res);
> +
> +	/* check again since we dropped the lock for the allocation */
> +	spin_lock(&ailp->ail_lock);
> +	if (ailp->ail_relog_tic) {
> +		xfs_log_ticket_get(ailp->ail_relog_tic);
> +		spin_unlock(&ailp->ail_lock);
> +		xfs_log_ticket_put(tic);
> +		return 0;
> +	}
> +
> +	/* attach and reserve space for the ->tr_relog ticket */
> +	ailp->ail_relog_tic = tic;
> +	tp->t_ticket->t_curr_res -= tic->t_curr_res;
> +	spin_unlock(&ailp->ail_lock);
> +
> +	return xfs_trans_roll(tpp);
> +}
> +
> +/*
> + * Release a reference to the relog ticket.
> + */
> +int
> +xfs_trans_ail_relog_put(
> +	struct xfs_mount	*mp)
> +{
> +	struct xfs_ail		*ailp = mp->m_ail;
> +	struct xlog_ticket	*tic;
> +
> +	spin_lock(&ailp->ail_lock);
> +	if (atomic_add_unless(&ailp->ail_relog_tic->t_ref, -1, 1)) {
> +		spin_unlock(&ailp->ail_lock);
> +		return 0;
> +	}
> +
> +	ASSERT(atomic_read(&ailp->ail_relog_tic->t_ref) == 1);
> +	tic = ailp->ail_relog_tic;
> +	ailp->ail_relog_tic = NULL;
> +	spin_unlock(&ailp->ail_lock);
> +
> +	xfs_log_done(mp, tic, NULL, false);
> +	return 0;
> +}
> +
>   int
>   xfs_trans_ail_init(
>   	xfs_mount_t	*mp)
> @@ -854,6 +942,7 @@ xfs_trans_ail_destroy(
>   {
>   	struct xfs_ail	*ailp = mp->m_ail;
>   
> +	ASSERT(ailp->ail_relog_tic == NULL);
>   	kthread_stop(ailp->ail_task);
>   	kmem_free(ailp);
>   }
> diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
> index 2e073c1c4614..839df6559b9f 100644
> --- a/fs/xfs/xfs_trans_priv.h
> +++ b/fs/xfs/xfs_trans_priv.h
> @@ -61,6 +61,7 @@ struct xfs_ail {
>   	int			ail_log_flush;
>   	struct list_head	ail_buf_list;
>   	wait_queue_head_t	ail_empty;
> +	struct xlog_ticket	*ail_relog_tic;
>   };
>   
>   /*
>