Re: [PATCH 04/18] xfs: clear log incompat feature bits when the log is idle

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

On 3/31/21 6:09 PM, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> When there are no ongoing transactions and the log contents have been
> checkpointed back into the filesystem, the log performs 'covering',
> which is to say that it log a dummy transaction to record the fact that
> the tail has caught up with the head.  This is a good time to clear log
> incompat feature flags, because they are flags that are temporarily set
> to limit the range of kernels that can replay a dirty log.
> 
> Since it's possible that some other higher level thread is about to
> start logging items protected by a log incompat flag, we create a rwsem
> so that upper level threads can coordinate this with the log.  It would
> probably be more performant to use a percpu rwsem, but the ability to
> /try/ taking the write lock during covering is critical, and percpu
> rwsems do not provide that.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
ok, makes sense
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>

> ---
>   fs/xfs/libxfs/xfs_shared.h |    6 +++++
>   fs/xfs/xfs_log.c           |   49 ++++++++++++++++++++++++++++++++++++++++++++
>   fs/xfs/xfs_log.h           |    3 +++
>   fs/xfs/xfs_log_priv.h      |    3 +++
>   fs/xfs/xfs_trans.c         |   14 +++++++++----
>   5 files changed, 71 insertions(+), 4 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_shared.h b/fs/xfs/libxfs/xfs_shared.h
> index 8c61a461bf7b..c7c9a0cebb04 100644
> --- a/fs/xfs/libxfs/xfs_shared.h
> +++ b/fs/xfs/libxfs/xfs_shared.h
> @@ -62,6 +62,12 @@ void	xfs_log_get_max_trans_res(struct xfs_mount *mp,
>   #define	XFS_TRANS_SB_DIRTY	0x02	/* superblock is modified */
>   #define	XFS_TRANS_PERM_LOG_RES	0x04	/* xact took a permanent log res */
>   #define	XFS_TRANS_SYNC		0x08	/* make commit synchronous */
> +/*
> + * This transaction uses a log incompat feature, which means that we must tell
> + * the log that we've finished using it at the transaction commit or cancel.
> + * Callers must call xlog_use_incompat_feat before setting this flag.
> + */
> +#define XFS_TRANS_LOG_INCOMPAT	0x10
>   #define XFS_TRANS_RESERVE	0x20    /* OK to use reserved data blocks */
>   #define XFS_TRANS_NO_WRITECOUNT 0x40	/* do not elevate SB writecount */
>   #define XFS_TRANS_RES_FDBLKS	0x80	/* reserve newly freed blocks */
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index cf73bc9f4d18..cb72be62da3e 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1335,6 +1335,32 @@ xfs_log_work_queue(
>   				msecs_to_jiffies(xfs_syncd_centisecs * 10));
>   }
>   
> +/*
> + * Clear the log incompat flags if we have the opportunity.
> + *
> + * This only happens if we're about to log the second dummy transaction as part
> + * of covering the log and we can get the log incompat feature usage lock.
> + */
> +static inline void
> +xlog_clear_incompat(
> +	struct xlog		*log)
> +{
> +	struct xfs_mount	*mp = log->l_mp;
> +
> +	if (!xfs_sb_has_incompat_log_feature(&mp->m_sb,
> +				XFS_SB_FEAT_INCOMPAT_LOG_ALL))
> +		return;
> +
> +	if (log->l_covered_state != XLOG_STATE_COVER_DONE2)
> +		return;
> +
> +	if (!down_write_trylock(&log->l_incompat_users))
> +		return;
> +
> +	xfs_clear_incompat_log_features(mp);
> +	up_write(&log->l_incompat_users);
> +}
> +
>   /*
>    * Every sync period we need to unpin all items in the AIL and push them to
>    * disk. If there is nothing dirty, then we might need to cover the log to
> @@ -1361,6 +1387,7 @@ xfs_log_worker(
>   		 * synchronously log the superblock instead to ensure the
>   		 * superblock is immediately unpinned and can be written back.
>   		 */
> +		xlog_clear_incompat(log);
>   		xfs_sync_sb(mp, true);
>   	} else
>   		xfs_log_force(mp, 0);
> @@ -1443,6 +1470,8 @@ xlog_alloc_log(
>   	}
>   	log->l_sectBBsize = 1 << log2_size;
>   
> +	init_rwsem(&log->l_incompat_users);
> +
>   	xlog_get_iclog_buffer_size(mp, log);
>   
>   	spin_lock_init(&log->l_icloglock);
> @@ -3933,3 +3962,23 @@ xfs_log_in_recovery(
>   
>   	return log->l_flags & XLOG_ACTIVE_RECOVERY;
>   }
> +
> +/*
> + * Notify the log that we're about to start using a feature that is protected
> + * by a log incompat feature flag.  This will prevent log covering from
> + * clearing those flags.
> + */
> +void
> +xlog_use_incompat_feat(
> +	struct xlog		*log)
> +{
> +	down_read(&log->l_incompat_users);
> +}
> +
> +/* Notify the log that we've finished using log incompat features. */
> +void
> +xlog_drop_incompat_feat(
> +	struct xlog		*log)
> +{
> +	up_read(&log->l_incompat_users);
> +}
> diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
> index 044e02cb8921..8b7d0a56cbf1 100644
> --- a/fs/xfs/xfs_log.h
> +++ b/fs/xfs/xfs_log.h
> @@ -145,4 +145,7 @@ bool	xfs_log_in_recovery(struct xfs_mount *);
>   
>   xfs_lsn_t xlog_grant_push_threshold(struct xlog *log, int need_bytes);
>   
> +void xlog_use_incompat_feat(struct xlog *log);
> +void xlog_drop_incompat_feat(struct xlog *log);
> +
>   #endif	/* __XFS_LOG_H__ */
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 1c6fdbf3d506..75702c4fa69c 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -436,6 +436,9 @@ struct xlog {
>   #endif
>   	/* log recovery lsn tracking (for buffer submission */
>   	xfs_lsn_t		l_recovery_lsn;
> +
> +	/* Users of log incompat features should take a read lock. */
> +	struct rw_semaphore	l_incompat_users;
>   };
>   
>   #define XLOG_BUF_CANCEL_BUCKET(log, blkno) \
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index eb2d8e2e5db6..e548d53c2091 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -71,6 +71,9 @@ xfs_trans_free(
>   	xfs_extent_busy_sort(&tp->t_busy);
>   	xfs_extent_busy_clear(tp->t_mountp, &tp->t_busy, false);
>   
> +	if (tp->t_flags & XFS_TRANS_LOG_INCOMPAT)
> +		xlog_drop_incompat_feat(tp->t_mountp->m_log);
> +
>   	trace_xfs_trans_free(tp, _RET_IP_);
>   	xfs_trans_clear_context(tp);
>   	if (!(tp->t_flags & XFS_TRANS_NO_WRITECOUNT))
> @@ -110,10 +113,13 @@ xfs_trans_dup(
>   	ASSERT(tp->t_flags & XFS_TRANS_PERM_LOG_RES);
>   	ASSERT(tp->t_ticket != NULL);
>   
> -	ntp->t_flags = XFS_TRANS_PERM_LOG_RES |
> -		       (tp->t_flags & XFS_TRANS_RESERVE) |
> -		       (tp->t_flags & XFS_TRANS_NO_WRITECOUNT) |
> -		       (tp->t_flags & XFS_TRANS_RES_FDBLKS);
> +	ntp->t_flags = tp->t_flags & (XFS_TRANS_PERM_LOG_RES |
> +				      XFS_TRANS_RESERVE |
> +				      XFS_TRANS_NO_WRITECOUNT |
> +				      XFS_TRANS_RES_FDBLKS |
> +				      XFS_TRANS_LOG_INCOMPAT);
> +	/* Give our LOG_INCOMPAT reference to the new transaction. */
> +	tp->t_flags &= ~XFS_TRANS_LOG_INCOMPAT;
>   	/* We gave our writer reference to the new transaction */
>   	tp->t_flags |= XFS_TRANS_NO_WRITECOUNT;
>   	ntp->t_ticket = xfs_log_ticket_get(tp->t_ticket);
>