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

[Jens Axboe]

On 03/22/2016 03:40 PM, Jens Axboe wrote:
> 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().

For block here, you mean the async work ending up doing 
wait_on_page_writeback() because we're doing WB_SYNC_ALL instead of 
WB_SYNC_NONE?

And if so:

> > 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.

why not have it do that?

--
Jens Axboe