linux-kernel - Re: [PATCH 2/6] writeback: wb_start_writeback() should use WB_SYNC_ALL for WB_REASON

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Message-ID: <56F1BBCC.8050004@fb.com>
Date:	Tue, 22 Mar 2016 15:40:28 -0600
From:	Jens Axboe <axboe@...com>
To:	Dave Chinner <david@...morbit.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-block@...r.kernel.org>
Subject: Re: [PATCH 2/6] writeback: wb_start_writeback() should use
 WB_SYNC_ALL for WB_REASON_SYNC

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