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

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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Jan Kara <[e-mail]>
Suggested-by: Jan Kara <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/fs-writeback.c

···
 
 /**
  * cgroup_writeback_umount - flush inode wb switches for umount
+ * @sb: target super_block
  *
  * This function is called when a super_block is about to be destroyed and
  * flushes in-flight inode wb switches.  An inode wb switch goes through
···
  * 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().

fs/super.c

···
 		sync_filesystem(sb);
 		sb->s_flags &= ~SB_ACTIVE;
 
-		cgroup_writeback_umount();
+		cgroup_writeback_umount(sb);
 
 		/* Evict all inodes with zero refcount. */
 		evict_inodes(sb);

include/linux/writeback.h

···
 			      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);
 
 /**
···
 {
 }
 
-static inline void cgroup_writeback_umount(void)
+static inline void cgroup_writeback_umount(struct super_block *sb)
 {
 }