linux-ext4 - Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

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Date:	Fri, 21 Jan 2011 19:49:55 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, tytso@....edu, djwong@...ibm.com, shli@...nel.org,
	neilb@...e.de, adilger.kernel@...ger.ca, jack@...e.cz,
	linux-kernel@...r.kernel.org, kmannth@...ibm.com, cmm@...ibm.com,
	linux-ext4@...r.kernel.org, rwheeler@...hat.com, hch@....de,
	josef@...hat.com
Subject: Re: [PATCH 3/3] block: reimplement FLUSH/FUA to support merge

On Fri, Jan 21 2011 at 10:59am -0500,
Tejun Heo <tj@...nel.org> wrote:

> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 8592869..cd73e93 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -1,6 +1,69 @@
>  /*
>   * Functions to sequence FLUSH and FUA writes.
> + *
> + * Copyright (C) 2011		Max Planck Institute for Gravitational Physics
> + * Copyright (C) 2011		Tejun Heo <tj@...nel.org>
> + *
> + * This file is released under the GPLv2.
> + *
> + * REQ_{FLUSH|FUA} requests are decomposed to sequences consisted of three
> + * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
> + * properties and hardware capability.
> + *
> + * If a request doesn't have data, only REQ_FLUSH makes sense, which
> + * indicates a simple flush request.  If there is data, REQ_FLUSH indicates
> + * that the device cache should be flushed before the data is executed, and
> + * REQ_FUA means that the data must be on non-volatile media on request
> + * completion.
> + *
> + * If the device doesn't have writeback cache, FLUSH and FUA don't make any
> + * difference.  The requests are either completed immediately if there's no
> + * data or executed as normal requests otherwise.

For devices without a writeback cache, I'm not seeing where pure flushes
are completed immediately.  But I do see where data is processed
directly in blk_insert_flush().


> -struct request *blk_do_flush(struct request_queue *q, struct request *rq)
> +/**
> + * blk_abort_flush - @q is being aborted, abort flush requests
      ^^^^^^^^^^^^^^^ 
Small comment nit, s/blk_abort_flush/blk_abort_flushes/

> + * @q: request_queue being aborted
> + *
> + * To be called from elv_abort_queue().  @q is being aborted.  Prepare all
> + * FLUSH/FUA requests for abortion.
> + *
> + * CONTEXT:
> + * spin_lock_irq(q->queue_lock)
> + */
> +void blk_abort_flushes(struct request_queue *q)
--
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