Re: [PATCH 2/6] writeback: wb_start_writeback() should use WB_SYNC_ALL for WB_REASON_SYNC

[Jens Axboe]

On 03/22/2016 03:34 PM, Dave Chinner wrote:
> On Tue, Mar 22, 2016 at 11:55:16AM -0600, Jens Axboe wrote:
> > If you call sync, the initial call to wakeup_flusher_threads() ends up
> > calling wb_start_writeback() with reason=WB_REASON_SYNC, but
> > wb_start_writeback() always uses WB_SYNC_NONE as the writeback mode.
> > Ensure that we use WB_SYNC_ALL for a sync operation.
>
> This seems wrong to me. We want background write to happen as
> quickly as possible and /not block/ when we first kick sync.

It's not going to block. wakeup_flusher_threads() async queues writeback 
work through wb_start_writeback().

> The latter blocking passes of sync use WB_SYNC_ALL to ensure that we
> block waiting for all remaining IO to be issued and waited on, but
> the background writeback doesn't need to do this.

That's fine, they can get to wait on the previously issued IO, which was 
properly submitted with WB_SYNC_ALL.

Maybe I'm missing your point?

--
Jens Axboe