[PATCH v9 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.

Use write_inode_now() with WB_SYNC_NONE to skip inodes already under
writeback, avoiding competing writeback passes from multiple callers.

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

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
---
 block/bdev.c           | 19 +++++++++++++++++++
 include/linux/blkdev.h |  4 ++++
 2 files changed, 23 insertions(+)

diff --git a/block/bdev.c b/block/bdev.c
index cd8323083740..8925f81f16ae 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -264,6 +264,25 @@ 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
+ *
+ * May sleep while submitting writeback; does not wait for I/O completion.
+ */
+void bdev_flush_by_dev(dev_t dev)
+{
+	struct block_device *bdev;
+
+	bdev = blkdev_get_no_open(dev, false);
+	if (!bdev)
+		return;
+
+	/* Use the I_SYNC check to avoid concurrent writeback. */
+	write_inode_now(BD_INODE(bdev), 0);
+	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..27afe3ff6fb5 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);
@@ -1726,6 +1727,9 @@ static inline int sync_blockdev_nowait(struct block_device *bdev)
 {
 	return 0;
 }
+static inline void bdev_flush_by_dev(dev_t dev)
+{
+}
 static inline void sync_bdevs(bool wait)
 {
 }
-- 
2.39.5