[PATCH v6 1/3] block: introduce bdev_flush_by_dev()

[Julian Sun <sunjunchao@bytedance.com>]

Add bdev_flush_by_dev() to submit page-cache writeback for a device
identified by dev_t without requiring callers to hold a persistent
device reference.

A subsequent patch will use this helper to flush foreign bdev mappings.

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
---
 block/bdev.c           | 24 ++++++++++++++++++++++++
 include/linux/blkdev.h |  1 +
 2 files changed, 25 insertions(+)

diff --git a/block/bdev.c b/block/bdev.c
index cd8323083740..da1fc56926fd 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -264,6 +264,30 @@ int sync_blockdev_nowait(struct block_device *bdev)
 }
 EXPORT_SYMBOL_GPL(sync_blockdev_nowait);
 
+/**
+ * bdev_flush_by_dev - attempt writeback of a block device's page cache
+ * @dev: target block device number
+ *
+ * Skip closed devices or devices whose open_mutex is busy.
+ * May sleep while submitting writeback; does not wait for all I/O to complete.
+ */
+void bdev_flush_by_dev(dev_t dev)
+{
+	struct block_device *bdev;
+
+	bdev = blkdev_get_no_open(dev, false);
+	if (!bdev)
+		return;
+
+	/* Prevent the device from closing between the check and writeback. */
+	if (mutex_trylock(&bdev->bd_disk->open_mutex)) {
+		if (atomic_read(&bdev->bd_openers))
+			sync_blockdev_nowait(bdev);
+		mutex_unlock(&bdev->bd_disk->open_mutex);
+	}
+	blkdev_put_no_open(bdev);
+}
+
 /*
  * Write out and wait upon all the dirty data associated with a block
  * device via its mapping.  Does not take the superblock lock.
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 4f7905c3412b..7a5f3e2f17d6 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1710,6 +1710,7 @@ void invalidate_bdev(struct block_device *bdev);
 int sync_blockdev(struct block_device *bdev);
 int sync_blockdev_range(struct block_device *bdev, loff_t lstart, loff_t lend);
 int sync_blockdev_nowait(struct block_device *bdev);
+void bdev_flush_by_dev(dev_t dev);
 void sync_bdevs(bool wait);
 void bdev_statx(const struct path *path, struct kstat *stat, u32 request_mask);
 void printk_all_partitions(void);
-- 
2.39.5