xfs: clear log incompat feature bits when the log is idlelog-use-incompat-features_2021-08-25

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>

3 files changed, 55 insertions, 0 deletions

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index c70161efe6bb..3cf265c5374e 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1363,6 +1363,32 @@ 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.
@@ -1388,6 +1414,7 @@ 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);
@@ -1475,6 +1502,8 @@ 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);
@@ -3974,3 +4003,23 @@ 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 813b972e9788..b274fb9dcd8d 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 f3e79a45d60a..6953f86f866c 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -456,6 +456,9 @@ 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) \