blkcg: update blkcg_print_stat() to handle larger outputs

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 <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 1f7127b03490db873a6a631a9ca17d349995ac81..e4715b35d42c30704309547b6bc051e9b738ee0f 100644 (file)
--- 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);
+                       }
                }
        }