linux-kernel - Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN

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Message-ID: <4C8CBC50.1070400@kernel.org>
Date:	Sun, 12 Sep 2010 13:41:04 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Tao Ma <tao.ma@...cle.com>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN_FLUSH

Hello,

On 09/12/2010 10:38 AM, Tao Ma wrote:
> According to Documentation/block/barrier.txt,
> QUEUE_ORDERED_DRAIN
>         Requests are ordered by draining the request queue and cache
>         flushing isn't needed.
> 
>         Sequence: drain => barrier
> 
> QUEUE_ORDERED_DRAIN_FLUSH
>         Requests are ordered by draining the request queue and both
>         pre-barrier and post-barrier cache flushings are needed.
> 
>         Sequence: drain => preflush => barrier => postflush
> 
> And for loop device, it call fsync in barrier request. See do_bio_filebacked in drivers/block/loop.c
> bool barrier = !!(bio->bi_rw & REQ_HARDBARRIER);
>                 struct file *file = lo->lo_backing_file;
> 
>                 if (barrier) {
>                         if (unlikely(!file->f_op->fsync)) {
>                                 ret = -EOPNOTSUPP;
>                                 goto out;
>                         }
> 
>                         ret = vfs_fsync(file, 0);
>                         if (unlikely(ret)) {
>                                 ret = -EIO;
>                                 goto out;
>                         }
>                 }
> 
> So actually the sync is done in barrier and no extra flush is needed?
> Or Do I misread the rules in barrier.txt?

Hmmm... maybe the doc was a bit confusing.  Any device which has
writeback cache should have FLUSH in the queue ordered configuration.
The loop device used vfs_fsync() for cache flushing and didn't support
ordered sequence.  As such, it should use draining for request
ordering and suports FLUSH, so the mode to use is DRAIN_FLUSH.

Thanks.

-- 
tejun
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