md-bitmap: create and destroy wb_info_pool with the change of backlog

Since we can enable write-behind mode by write backlog node,
so create wb_info_pool if the mode is just enabled, also call
call md_bitmap_update_sb to make user aware the write-behind
mode is enabled. Conversely, wb_info_pool should be destroyed
when write-behind mode is disabled.

Beside above, it is better to update bitmap sb if we change
the number of max_write_behind.
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/md-bitmap.c

@@ -2462,12 +2462,26 @@ static ssize_t
 backlog_store(struct mddev *mddev, const char *buf, size_t len)
 {
 	unsigned long backlog;
+	unsigned long old_mwb = mddev->bitmap_info.max_write_behind;
 	int rv = kstrtoul(buf, 10, &backlog);
 	if (rv)
 		return rv;
 	if (backlog > COUNTER_MAX)
 		return -EINVAL;
 	mddev->bitmap_info.max_write_behind = backlog;
+	if (!backlog && mddev->wb_info_pool) {
+		/* wb_info_pool is not needed if backlog is zero */
+		mempool_destroy(mddev->wb_info_pool);
+		mddev->wb_info_pool = NULL;
+	} else if (backlog && !mddev->wb_info_pool) {
+		/* wb_info_pool is needed since backlog is not zero */
+		struct md_rdev *rdev;
+
+		rdev_for_each(rdev, mddev)
+			mddev_create_wb_pool(mddev, rdev, false);
+	}
+	if (old_mwb != backlog)
+		md_bitmap_update_sb(mddev->bitmap);
 	return len;
 }