[PATCH 3/8] bdi: add a ->dev_name field to struct backing_dev_info

[Christoph Hellwig <hch@xxxxxx>]

Cache a copy of the name for the life time of the backing_dev_info
structure so that we can reference it even after unregistering.

Fixes: 68f23b89067f ("memcg: fix a crash in wb_workfn when a device disappears")
Reported-by: Yufen Yu <yuyufen@xxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 include/linux/backing-dev-defs.h |  1 +
 mm/backing-dev.c                 | 13 ++++++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 4fc87dee005a..249590bcccf7 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -220,6 +220,7 @@ struct backing_dev_info {
 	wait_queue_head_t wb_waitq;
 
 	struct device *dev;
+	const char *dev_name;
 	struct device *owner;
 
 	struct timer_list laptop_mode_wb_timer;
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c2c44c89ee5d..4f6c05df72f9 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -938,9 +938,15 @@ int bdi_register_va(struct backing_dev_info *bdi, const char *fmt, va_list args)
 	if (bdi->dev)	/* The driver needs to use separate queues per device */
 		return 0;
 
-	dev = device_create_vargs(bdi_class, NULL, MKDEV(0, 0), bdi, fmt, args);
-	if (IS_ERR(dev))
+	bdi->dev_name = kvasprintf(GFP_KERNEL, fmt, args);
+	if (!bdi->dev_name)
+		return -ENOMEM;
+
+	dev = device_create(bdi_class, NULL, MKDEV(0, 0), bdi, bdi->dev_name);
+	if (IS_ERR(dev)) {
+		kfree(bdi->dev_name);
 		return PTR_ERR(dev);
+	}
 
 	cgwb_bdi_register(bdi);
 	bdi->dev = dev;
@@ -1034,6 +1040,7 @@ static void release_bdi(struct kref *ref)
 	WARN_ON_ONCE(bdi->dev);
 	wb_exit(&bdi->wb);
 	cgwb_bdi_exit(bdi);
+	kfree(bdi->dev_name);
 	kfree(bdi);
 }
 
@@ -1047,7 +1054,7 @@ const char *bdi_dev_name(struct backing_dev_info *bdi)
 {
 	if (!bdi || !bdi->dev)
 		return bdi_unknown_name;
-	return dev_name(bdi->dev);
+	return bdi->dev_name;
 }
 EXPORT_SYMBOL_GPL(bdi_dev_name);
 
-- 
2.25.1