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

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Message-ID: <20160322213427.GR11812@dastard>
Date:	Wed, 23 Mar 2016 08:34:27 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jens Axboe <axboe@...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 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.

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.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com