Re: [PATCH v22 02/16] xfs: clear log incompat feature bits when the log is idle

[Chandan Babu R <chandanrlinux@gmail.com>]

On 27 Jul 2021 at 11:50, Allison Henderson 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.
>

Looks good to me.

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
> Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
> ---
>  fs/xfs/xfs_log.c      | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_log.h      |  3 +++
>  fs/xfs/xfs_log_priv.h |  3 +++
>  3 files changed, 55 insertions(+)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 9254405..c58a0d7 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1338,6 +1338,32 @@ xfs_log_work_queue(
>  }
>  
>  /*
> + * 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
>   * indicate that the filesystem is idle.
> @@ -1363,6 +1389,7 @@ 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);
> @@ -1450,6 +1477,8 @@ 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);
> @@ -3895,3 +3924,23 @@ 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 813b972..b274fb9 100644
> --- a/fs/xfs/xfs_log.h
> +++ b/fs/xfs/xfs_log.h
> @@ -142,4 +142,7 @@ 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 4c41bbfa..c507041 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -449,6 +449,9 @@ struct xlog {
>  	xfs_lsn_t		l_recovery_lsn;
>  
>  	uint32_t		l_iclog_roundoff;/* padding roundoff */
> +
> +	/* Users of log incompat features should take a read lock. */
> +	struct rw_semaphore	l_incompat_users;
>  };
>  
>  #define XLOG_BUF_CANCEL_BUCKET(log, blkno) \


-- 
chandan