[PATCH v2] fs: don't flush in-flight wb switches for superblocks without cgroup writeback

[Haifeng Xu <haifeng.xu@xxxxxxxxxx>]

When deactivating any type of superblock, it had to wait for the in-flight
wb switches to be completed. wb switches are executed in inode_switch_wbs_work_fn()
which needs to acquire the wb_switch_rwsem and races against sync_inodes_sb().
If there are too much dirty data in the superblock, the waiting time may increase
significantly.

For superblocks without cgroup writeback such as tmpfs, they have nothing to
do with the wb swithes, so the flushing can be avoided.

Signed-off-by: Haifeng Xu <haifeng.xu@xxxxxxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxx>
---
Changes since v1:
- do the check in cgroup_writeback_umount().
- check the capabilities of bdi.
---
 fs/fs-writeback.c         | 6 +++++-
 fs/super.c                | 2 +-
 include/linux/writeback.h | 4 ++--
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 92a5b8283528..09facd4356d9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1140,8 +1140,12 @@ int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
  * rare occurrences and synchronize_rcu() can take a while, perform
  * flushing iff wb switches are in flight.
  */
-void cgroup_writeback_umount(void)
+void cgroup_writeback_umount(struct super_block *sb)
 {
+
+	if (!(sb->s_bdi->capabilities & BDI_CAP_WRITEBACK))
+		return;
+
 	/*
 	 * SB_ACTIVE should be reliably cleared before checking
 	 * isw_nr_in_flight, see generic_shutdown_super().
diff --git a/fs/super.c b/fs/super.c
index 095ba793e10c..acc16450da0e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -621,7 +621,7 @@ void generic_shutdown_super(struct super_block *sb)
 		sync_filesystem(sb);
 		sb->s_flags &= ~SB_ACTIVE;
 
-		cgroup_writeback_umount();
+		cgroup_writeback_umount(sb);
 
 		/* Evict all inodes with zero refcount. */
 		evict_inodes(sb);
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 112d806ddbe4..d78d3dce4ede 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -217,7 +217,7 @@ void wbc_account_cgroup_owner(struct writeback_control *wbc, struct page *page,
 			      size_t bytes);
 int cgroup_writeback_by_id(u64 bdi_id, int memcg_id,
 			   enum wb_reason reason, struct wb_completion *done);
-void cgroup_writeback_umount(void);
+void cgroup_writeback_umount(struct super_block *sb);
 bool cleanup_offline_cgwb(struct bdi_writeback *wb);
 
 /**
@@ -324,7 +324,7 @@ static inline void wbc_account_cgroup_owner(struct writeback_control *wbc,
 {
 }
 
-static inline void cgroup_writeback_umount(void)
+static inline void cgroup_writeback_umount(struct super_block *sb)
 {
 }
 
-- 
2.25.1