xfs: periodically relog deferred intent items

There's a subtle design flaw in the deferred log item code that can lead
to pinning the log tail.  Taking up the defer ops chain examples from
the previous commit, we can get trapped in sequences like this:

Caller hands us a transaction t0 with D0-D3 attached.  The defer ops
chain will look like the following if the transaction rolls succeed:

t1: D0(t0), D1(t0), D2(t0), D3(t0)
t2: d4(t1), d5(t1), D1(t0), D2(t0), D3(t0)
t3: d5(t1), D1(t0), D2(t0), D3(t0)
...
t9: d9(t7), D3(t0)
t10: D3(t0)
t11: d10(t10), d11(t10)
t12: d11(t10)

In transaction 9, we finish d9 and try to roll to t10 while holding onto
an intent item for D3 that we logged in t0.

The previous commit changed the order in which we place new defer ops in
the defer ops processing chain to reduce the maximum chain length.  Now
make xfs_defer_finish_noroll capable of relogging the entire chain
periodically so that we can always move the log tail forward.  Most
chains will never get relogged, except for operations that generate very
long chains (large extents containing many blocks with different sharing
levels) or are on filesystems with small logs and a lot of ongoing
metadata updates.

Callers are now required to ensure that the transaction reservation is
large enough to handle logging done items and new intent items for the
maximum possible chain length.  Most callers are careful to keep the
chain lengths low, so the overhead should be minimal.

The decision to relog an intent item is made based on whether the intent
was logged in a previous checkpoint, since there's no point in relogging
an intent into the same checkpoint.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
index 3aaf8f6f05a4..fbe64933bcc9 100644
--- a/fs/xfs/libxfs/xfs_defer.c
+++ b/fs/xfs/libxfs/xfs_defer.c
@@ -17,6 +17,7 @@
 #include "xfs_inode_item.h"
 #include "xfs_trace.h"
 #include "xfs_icache.h"
+#include "xfs_log.h"
 
 /*
  * Deferred Operations in XFS
@@ -346,6 +347,42 @@ xfs_defer_cancel_list(
 }
 
 /*
+ * Prevent a log intent item from pinning the tail of the log by logging a
+ * done item to release the intent item; and then log a new intent item.
+ * The caller should provide a fresh transaction and roll it after we're done.
+ */
+static int
+xfs_defer_relog(
+	struct xfs_trans		**tpp,
+	struct list_head		*dfops)
+{
+	struct xfs_defer_pending	*dfp;
+
+	ASSERT((*tpp)->t_flags & XFS_TRANS_PERM_LOG_RES);
+
+	list_for_each_entry(dfp, dfops, dfp_list) {
+		/*
+		 * If the log intent item for this deferred op is not a part of
+		 * the current log checkpoint, relog the intent item to keep
+		 * the log tail moving forward.  We're ok with this being racy
+		 * because an incorrect decision means we'll be a little slower
+		 * at pushing the tail.
+		 */
+		if (dfp->dfp_intent == NULL ||
+		    xfs_log_item_in_current_chkpt(dfp->dfp_intent))
+			continue;
+
+		trace_xfs_defer_relog_intent((*tpp)->t_mountp, dfp);
+		XFS_STATS_INC((*tpp)->t_mountp, defer_relog);
+		dfp->dfp_intent = xfs_trans_item_relog(dfp->dfp_intent, *tpp);
+	}
+
+	if ((*tpp)->t_flags & XFS_TRANS_DIRTY)
+		return xfs_defer_trans_roll(tpp);
+	return 0;
+}
+
+/*
  * Log an intent-done item for the first pending intent, and finish the work
  * items.
  */
@@ -431,6 +468,11 @@ xfs_defer_finish_noroll(
 		if (error)
 			goto out_shutdown;
 
+		/* Possibly relog intent items to keep the log moving. */
+		error = xfs_defer_relog(tp, &dop_pending);
+		if (error)
+			goto out_shutdown;
+
 		dfp = list_first_entry(&dop_pending, struct xfs_defer_pending,
 				       dfp_list);
 		error = xfs_defer_finish_one(*tp, dfp);