Re: [PATCH] bdi_sync_writeback should WB_SYNC_NONE first

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 25 Sep 2009 10:10:14 -0400 Chris Mason <chris.mason@oracle.com> wrote:

> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 8e1e5e1..27f8e0e 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -225,7 +225,7 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
>  {
>  	struct wb_writeback_args args = {
>  		.sb		= sb,
> -		.sync_mode	= WB_SYNC_ALL,
> +		.sync_mode	= WB_SYNC_NONE,
>  		.nr_pages	= LONG_MAX,
>  		.range_cyclic	= 0,
>  	};
> @@ -236,6 +236,13 @@ static void bdi_sync_writeback(struct backing_dev_info *bdi,
>  
>  	bdi_queue_work(bdi, &work);
>  	bdi_wait_on_work_clear(&work);
> +
> +	args.sync_mode = WB_SYNC_ALL;
> +	args.nr_pages = LONG_MAX;
> +
> +	work.state = WS_USED | WS_ONSTACK;
> +	bdi_queue_work(bdi, &work);
> +	bdi_wait_on_work_clear(&work);
>  }

Those LONG_MAX's are a worry.  What prevents a very long
almost-livelock from occurring if userspace is concurrently dirtying
pagecache at a high rate?