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

From: Jens Axboe
Date: Tue Mar 22 2016 - 17:40:36 EST

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