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Date: Wed, 20 Dec 2023 22:12:49 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Song Liu <song@kernel.org>
Cc: linux-block@vger.kernel.org, linux-raid@vger.kernel.org,
	axboe@kernel.dk, kent.overstreet@linux.dev,
	janpieter.sollie@edpnet.be, colyli@suse.de, bagasdotme@gmail.com
Subject: Re: [PATCH 1/2] block: Check REQ_OP_FLUSH in op_is_flush()
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References: <20231221012715.3048221-1-song@kernel.org>
 <20231221012715.3048221-2-song@kernel.org>
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On Wed, Dec 20, 2023 at 05:27:14PM -0800, Song Liu wrote:
> Upper layer (fs, etc.) may issue flush with something like:
> 
>   bio_reset(bio, bdev, REQ_OP_FLUSH);
>   bio->bi_end_io = xxx;
>   submit_bio(bio);

No, they can't.  REQ_OP_FLUSH is currently only used by request
based drivers and only generated by the flush state machine.

(Not that I particularly like this wart, I'd much prefer file systems
to submit a REQ_OP_FLUSH than an empty write with a preflush flag,
but that's not a trivial change).