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

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On 7/27/21 5:46 AM, Chandan Babu R wrote:
On 27 Jul 2021 at 11:50, Allison Henderson wrote:
From: "Darrick J. Wong" <djwong@xxxxxxxxxx>

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@xxxxxxxxx>
Thank you!

Allison


Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
Signed-off-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
---
  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) \





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