[RFC PATCH] blkcg: rewind seq_file if no stats

[Khazhismel Kumykov <khazhy@google.com>]

In lieu of get_seq_buf + seq_commit, provide a way to "undo" writes if
we use seq_printf

Fixes: 252c651a4c85 ("blk-cgroup: stop using seq_get_buf")

Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
---
 block/blk-cgroup.c       |  5 +++++
 fs/seq_file.c            | 14 ++++++++++++++
 include/linux/seq_file.h |  2 ++
 3 files changed, 21 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 8dfe62786cd5..50043a742c48 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -909,6 +909,7 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 	const char *dname;
 	unsigned seq;
 	int i;
+	int scookie;
 
 	if (!blkg->online)
 		return;
@@ -917,6 +918,8 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 	if (!dname)
 		return;
 
+	scookie = seq_checkpoint(s);
+
 	seq_printf(s, "%s ", dname);
 
 	do {
@@ -956,6 +959,8 @@ static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
 
 	if (has_stats)
 		seq_printf(s, "\n");
+	else
+		seq_restore(s, scookie);
 }
 
 static int blkcg_print_stat(struct seq_file *sf, void *v)
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 7ab8a58c29b6..c3ec6b57334e 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -408,6 +408,20 @@ void seq_printf(struct seq_file *m, const char *f, ...)
 }
 EXPORT_SYMBOL(seq_printf);
 
+int seq_checkpoint(struct seq_file *m)
+{
+	return m->count;
+}
+EXPORT_SYMBOL(seq_checkpoint);
+
+void seq_restore(struct seq_file *m, int count)
+{
+	if (WARN_ON_ONCE(count > m->count || count > m->size))
+		return;
+	m->count = count;
+}
+EXPORT_SYMBOL(seq_restore);
+
 #ifdef CONFIG_BINARY_PRINTF
 void seq_bprintf(struct seq_file *m, const char *f, const u32 *binary)
 {
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 60820ab511d2..d3a05f7c2750 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -117,6 +117,8 @@ __printf(2, 0)
 void seq_vprintf(struct seq_file *m, const char *fmt, va_list args);
 __printf(2, 3)
 void seq_printf(struct seq_file *m, const char *fmt, ...);
+int seq_checkpoint(struct seq_file *m);
+void seq_restore(struct seq_file *m, int count);
 void seq_putc(struct seq_file *m, char c);
 void seq_puts(struct seq_file *m, const char *s);
 void seq_put_decimal_ull_width(struct seq_file *m, const char *delimiter,
-- 
2.36.0.550.gb090851708-goog