Re: [PATCH 3/4] writeback: only use bdi_writeback_all() for WB_SYNC_NONE writeout

[Christoph Hellwig <hch@infradead.org>]

> -static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
> +static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> +				 struct writeback_control *wbc)
>  {
>  	struct bdi_work *work;
>  
> @@ -195,7 +196,7 @@ static struct bdi_work *bdi_alloc_work(struct writeback_control *wbc)
>  	if (work)
>  		bdi_work_init(work, wbc);
>  
> -	return work;
> +	bdi_queue_work(bdi, work);

This is now the only caller of bdi_queue_work that has a NULL work
argument.  I would recommend removing the !work half of bdi_queue_work
and just inline it into this function (or make it a separate helper).

>  /*
> + * Schedule writeback for all backing devices. Can only be used for
> + * WB_SYNC_NONE writeback, WB_SYNC_ALL should use bdi_start_writeback()
> + * and pass in the superblock.
>   */
>  static void bdi_writeback_all(struct writeback_control *wbc)
>  {
>  	struct backing_dev_info *bdi;
>  
> +	WARN_ON(wbc->sync_mode == WB_SYNC_ALL);
> +
>  	spin_lock(&bdi_lock);
>  
>  	list_for_each_entry(bdi, &bdi_list, bdi_list) {
>  		if (!bdi_has_dirty_io(bdi))
>  			continue;
>  
> +		bdi_alloc_queue_work(bdi, wbc);
>  	}


Much nicer than before.  Could be even further simplified to:

		if (bdi_has_dirty_io(bdi))
			bdi_alloc_queue_work(bdi, wbc);

> @@ -1157,6 +1115,7 @@ long sync_inodes_sb(struct super_block *sb)
>  {
>  	struct writeback_control wbc = {
>  		.sb		= sb,
> +		.bdi		= sb->s_bdi,
>  		.sync_mode	= WB_SYNC_ALL,
>  		.range_start	= 0,
>  		.range_end	= LLONG_MAX,
> @@ -1164,7 +1123,7 @@ long sync_inodes_sb(struct super_block *sb)
>  	long nr_to_write = LONG_MAX; /* doesn't actually matter */
>  
>  	wbc.nr_to_write = nr_to_write;
> -	bdi_writeback_all(&wbc);
> +	bdi_start_writeback(&wbc);

So here we have a WB_SYNC_ALL caller of bdi_writeback_all and the
only other caller is WB_SYNC_NONE.  Given that after patch two those
are entirely different codepathes in bdi_start_writeback I would just
split bdi_start_writeback into two separate functions.