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

[Tejun Heo <tj@kernel.org>]

Hello, Julian.

On Thu, Sep 17, 2026 at 02:57:57PM +0800, Julian Sun wrote:
> +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);
> +}

This holds the disk's open_mutex across the whole writepages pass, so every
throttle-triggered flush blocks opens, closes and partition rescans on the
disk for as long as submission takes, which can be a while on a congested
device. bdev_release() syncs before taking the mutex for the same reason.

I don't think the mutex or the openers check is needed. If the device is
closed underneath, the last close syncs and truncates the mapping and
truncate waits for writeback, so a racing no-wait flush either finishes
first or finds nothing left. Holding just the reference from
blkdev_get_no_open() should be enough.

> +void bdev_flush_by_dev(dev_t dev);

The neighbors have !CONFIG_BLOCK stubs. The only caller is under
CGROUP_WRITEBACK so it builds either way, just noting for consistency.

Thanks.

--
tejun