The switch to rhashtables (commit 88ffbf3e03) broke the debugfs glock dump (/sys/kernel/debug/gfs2/<device>/glocks) for dumps bigger than a single buffer: the right function for restarting an rhashtable iteration from the beginning of the hash table is rhashtable_walk_enter; rhashtable_walk_stop + rhashtable_walk_start will just resume from the current position.

The upstream commit doesn't directly apply to 4.9.y because 4.9.y doesn't have the following mainline commits:

92ecd73a887c4a2b94daf5fc35179d75d1c4ef95
gfs2: Deduplicate gfs2_{glocks,glstats}_open
cc37a62785a584f4875788689f3fd1fa6e4eb291
gfs2: Replace rhashtable_walk_init with rhashtable_walk_enter

Signed-off-by Andreas Gruenbacher <[email protected]>

Signed-off-by Bob Peterson <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 fs/gfs2/glock.c |   16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

fs/gfs2/glock.c +5 ~~-11~~

@@ -1836,13 +1836,9 @@ static void *gfs2_glock_seq_start(struct
 {
 	struct gfs2_glock_iter *gi = seq->private;
 	loff_t n = *pos;
-	int ret;
 
-	if (gi->last_pos <= *pos)
-		n = (*pos - gi->last_pos);
-
-	ret = rhashtable_walk_start(&gi->hti);
-	if (ret)
+	rhashtable_walk_enter(&gl_hash_table, &gi->hti);
+	if (rhashtable_walk_start(&gi->hti) != 0)
 		return NULL;
 
 	do {

@@ -1850,6 +1846,7 @@ static void *gfs2_glock_seq_start(struct
 	} while (gi->gl && n--);
 
 	gi->last_pos = *pos;
+
 	return gi->gl;
 }

@@ -1861,6 +1858,7 @@ static void *gfs2_glock_seq_next(struct
 	(*pos)++;
 	gi->last_pos = *pos;
 	gfs2_glock_iter_next(gi);
+
 	return gi->gl;
 }

@@ -1870,6 +1868,7 @@ static void gfs2_glock_seq_stop(struct s
 
 	gi->gl = NULL;
 	rhashtable_walk_stop(&gi->hti);
+	rhashtable_walk_exit(&gi->hti);
 }
 
 static int gfs2_glock_seq_show(struct seq_file *seq, void *iter_ptr)

@@ -1932,12 +1931,10 @@ static int gfs2_glocks_open(struct inode
 		struct gfs2_glock_iter *gi = seq->private;
 
 		gi->sdp = inode->i_private;
-		gi->last_pos = 0;
 		seq->buf = kmalloc(GFS2_SEQ_GOODSIZE, GFP_KERNEL | __GFP_NOWARN);
 		if (seq->buf)
 			seq->size = GFS2_SEQ_GOODSIZE;
 		gi->gl = NULL;
-		ret = rhashtable_walk_init(&gl_hash_table, &gi->hti, GFP_KERNEL);
 	}
 	return ret;
 }

@@ -1948,7 +1945,6 @@ static int gfs2_glocks_release(struct in
 	struct gfs2_glock_iter *gi = seq->private;
 
 	gi->gl = NULL;
-	rhashtable_walk_exit(&gi->hti);
 	return seq_release_private(inode, file);
 }

@@ -1960,12 +1956,10 @@ static int gfs2_glstats_open(struct inod
 		struct seq_file *seq = file->private_data;
 		struct gfs2_glock_iter *gi = seq->private;
 		gi->sdp = inode->i_private;
-		gi->last_pos = 0;
 		seq->buf = kmalloc(GFS2_SEQ_GOODSIZE, GFP_KERNEL | __GFP_NOWARN);
 		if (seq->buf)
 			seq->size = GFS2_SEQ_GOODSIZE;
 		gi->gl = NULL;
-		ret = rhashtable_walk_init(&gl_hash_table, &gi->hti, GFP_KERNEL);
 	}
 	return ret;
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/gfs2-fix-debugfs-glocks-dump.patch queue-4.9/vfs-return-enxio-for-negative-seek_hole-seek_data-offsets.patch