writeback, cgroup: split out the functional part of inode_switch_wbs_work_fn()

Split out the functional part of the inode_switch_wbs_work_fn() function
as inode_do switch_wbs() to reuse it later for switching inodes attached
to dying cgwbs.

This commit doesn't bring any functional changes.

Link: https://lkml.kernel.org/r/20210608230225.2078447-7-guro@fb.comSigned-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c

@@ -351,15 +351,12 @@ static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi)
 	up_write(&bdi->wb_switch_rwsem);
 }
 
-static void inode_switch_wbs_work_fn(struct work_struct *work)
+static void inode_do_switch_wbs(struct inode *inode,
+				struct bdi_writeback *new_wb)
 {
-	struct inode_switch_wbs_context *isw =
-		container_of(to_rcu_work(work), struct inode_switch_wbs_context, work);
-	struct inode *inode = isw->inode;
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	struct address_space *mapping = inode->i_mapping;
 	struct bdi_writeback *old_wb = inode->i_wb;
-	struct bdi_writeback *new_wb = isw->new_wb;
 	XA_STATE(xas, &mapping->i_pages, 0);
 	struct page *page;
 	bool switched = false;
@@ -470,11 +467,17 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 		wb_wakeup(new_wb);
 		wb_put(old_wb);
 	}
-	wb_put(new_wb);
+}
 
-	iput(inode);
-	kfree(isw);
+static void inode_switch_wbs_work_fn(struct work_struct *work)
+{
+	struct inode_switch_wbs_context *isw =
+		container_of(to_rcu_work(work), struct inode_switch_wbs_context, work);
 
+	inode_do_switch_wbs(isw->inode, isw->new_wb);
+	wb_put(isw->new_wb);
+	iput(isw->inode);
+	kfree(isw);
 	atomic_dec(&isw_nr_in_flight);
 }