btrfs: simplify ->flush_bio handling

Use and embedded bios that is initialized when used instead of
bio_kmalloc plus bio_reset.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220406061228.410163-2-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/btrfs/disk-io.c

@@ -4226,6 +4226,7 @@ static int wait_dev_supers(struct btrfs_device *device, int max_mirrors)
  */
 static void btrfs_end_empty_barrier(struct bio *bio)
 {
+	bio_uninit(bio);
 	complete(bio->bi_private);
 }
 
@@ -4235,7 +4236,7 @@ static void btrfs_end_empty_barrier(struct bio *bio)
  */
 static void write_dev_flush(struct btrfs_device *device)
 {
-	struct bio *bio = device->flush_bio;
+	struct bio *bio = &device->flush_bio;
 
 #ifndef CONFIG_BTRFS_FS_CHECK_INTEGRITY
 	/*
@@ -4253,7 +4254,8 @@ static void write_dev_flush(struct btrfs_device *device)
 		return;
 #endif
 
-	bio_reset(bio, device->bdev, REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH);
+	bio_init(bio, device->bdev, NULL, 0,
+		 REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH);
 	bio->bi_end_io = btrfs_end_empty_barrier;
 	init_completion(&device->flush_wait);
 	bio->bi_private = &device->flush_wait;
@@ -4267,7 +4269,7 @@ static void write_dev_flush(struct btrfs_device *device)
  */
 static blk_status_t wait_dev_flush(struct btrfs_device *device)
 {
-	struct bio *bio = device->flush_bio;
+	struct bio *bio = &device->flush_bio;
 
 	if (!test_bit(BTRFS_DEV_STATE_FLUSH_SENT, &device->dev_state))
 		return BLK_STS_OK;

fs/btrfs/volumes.c

@@ -405,7 +405,6 @@ void btrfs_free_device(struct btrfs_device *device)
 	WARN_ON(!list_empty(&device->post_commit_list));
 	rcu_string_free(device->name);
 	extent_io_tree_release(&device->alloc_state);
-	bio_put(device->flush_bio);
 	btrfs_destroy_dev_zone_info(device);
 	kfree(device);
 }
@@ -6949,16 +6948,6 @@ struct btrfs_device *btrfs_alloc_device(struct btrfs_fs_info *fs_info,
 	if (!dev)
 		return ERR_PTR(-ENOMEM);
 
-	/*
-	 * Preallocate a bio that's always going to be used for flushing device
-	 * barriers and matches the device lifespan
-	 */
-	dev->flush_bio = bio_kmalloc(GFP_KERNEL, 0);
-	if (!dev->flush_bio) {
-		kfree(dev);
-		return ERR_PTR(-ENOMEM);
-	}
-
 	INIT_LIST_HEAD(&dev->dev_list);
 	INIT_LIST_HEAD(&dev->dev_alloc_list);
 	INIT_LIST_HEAD(&dev->post_commit_list);

fs/btrfs/volumes.h

@@ -121,8 +121,8 @@ struct btrfs_device {
 	/* bytes used on the current transaction */
 	u64 commit_bytes_used;
 
-	/* for sending down flush barriers */
-	struct bio *flush_bio;
+	/* Bio used for flushing device barriers */
+	struct bio flush_bio;
 	struct completion flush_wait;
 
 	/* per-device scrub information */