Subject: Re: [PATCH 2/6] writeback: wb_start_writeback() should use
 WB_SYNC_ALL for WB_REASON_SYNC
To: Dave Chinner <david@fromorbit.com>
References: <1458669320-6819-1-git-send-email-axboe@fb.com>
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 <20160322213427.GR11812@dastard> <56F1BBCC.8050004@fb.com>
CC: <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <linux-block@vger.kernel.org>
From: Jens Axboe <axboe@fb.com>
Message-ID: <56F1BE45.5040906@fb.com>
Date: Tue, 22 Mar 2016 15:51:01 -0600
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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