Re: [PATCH 2/8] writeback: move dirty inodes from super_block to backing_dev_info

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Sep 03 2009, Christoph Hellwig wrote:
> Clean bdi_start_writeback up a bit:
> 
> 
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c	2009-09-03 23:50:12.880832607 -0300
> +++ linux-2.6/fs/fs-writeback.c	2009-09-03 23:54:42.372832985 -0300
> @@ -70,13 +70,6 @@ static inline void bdi_work_init(struct 
>  	work->state = WS_USED;
>  }
>  
> -static inline void bdi_work_init_on_stack(struct bdi_work *work,
> -					  struct writeback_control *wbc)
> -{
> -	bdi_work_init(work, wbc);
> -	work->state |= WS_ONSTACK;
> -}
> -
>  /**
>   * writeback_in_progress - determine whether there is writeback in progress
>   * @bdi: the device's backing_dev_info structure.
> @@ -207,35 +200,26 @@ static struct bdi_work *bdi_alloc_work(s
>  
>  void bdi_start_writeback(struct writeback_control *wbc)
>  {
> -	const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
> -	struct bdi_work work_stack, *work = NULL;
> -
> -	if (!must_wait)
> -		work = bdi_alloc_work(wbc);
> +	struct bdi_work work;
>  
> -	if (!work) {
> -		work = &work_stack;
> -		bdi_work_init_on_stack(work, wbc);
> +	if (wbc->sync_mode != WB_SYNC_ALL) {
> +		struct bdi_work *w = bdi_alloc_work(wbc);
> +		if (w) {
> +			bdi_queue_work(wbc->bdi, w);
> +			return;
> +		}
>  	}
>  
> -	bdi_queue_work(wbc->bdi, work);
> +	bdi_work_init(&work, wbc);
> +	work.state |= WS_ONSTACK;
> +	bdi_queue_work(wbc->bdi, &work);
>  
>  	/*
>  	 * If the sync mode is WB_SYNC_ALL, block waiting for the work to
> -	 * complete. If not, we only need to wait for the work to be started,
> -	 * if we allocated it on-stack. We use the same mechanism, if the
> -	 * wait bit is set in the bdi_work struct, then threads will not
> -	 * clear pending until after they are done.
> -	 *
> -	 * Note that work == &work_stack if must_wait is true, so we don't
> -	 * need to do call_rcu() here ever, since the completion path will
> -	 * have done that for us.
> +	 * complete.
>  	 */
> -	if (must_wait || work == &work_stack) {
> -		bdi_wait_on_work_clear(work);
> -		if (work != &work_stack)
> -			call_rcu(&work->rcu_head, bdi_work_free);
> -	}
> +	if (wbc->sync_mode == WB_SYNC_ALL)
> +		bdi_wait_on_work_clear(&work);
>  }

That doesn't work, you have to wait for on-stack work. So either we just
punt and not do anything for WB_SYNC_NONE if the allocation fails, or we
punt to stack and do the wait. Since it's a cleaning action and
allocation fails, falling back to the stack and waiting seems like the
most appropriate choice.

-- 
Jens Axboe