Depending on the number of devices, blkcg stats can go over the
default seqfile buf size.  seqfile normally retries with a larger
buffer but since the ->pd_stat() addition, blkcg_print_stat() doesn't
tell seqfile that overflow has happened and the output gets printed
truncated.  Fix it by calling seq_commit() w/ -1 on possible
overflows.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 903d23f0a354 ("blk-cgroup: allow controllers to output their own stats")
Cc: stable@vger.kernel.org # v4.19+
Cc: Josef Bacik <redacted>
---
 block/blk-cgroup.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 1f7127b03490..e4715b35d42c 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c

@@ -1006,8 +1006,12 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 		}
 next:
 		if (has_stats) {
-			off += scnprintf(buf+off, size-off, "\n");
-			seq_commit(sf, off);
+			if (off < size - 1) {
+				off += scnprintf(buf+off, size-off, "\n");
+				seq_commit(sf, off);
+			} else {
+				seq_commit(sf, -1);
+			}
 		}
 	}
 

-- 
2.17.1