linux-kernel - Question about REQ_FLUSH and bios with data

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Date:	Fri, 9 Aug 2013 14:09:34 -0500
From:	scameron@...rdog.cce.hp.com
To:	linux-kernel@...r.kernel.org
Cc:	jens.axboe@...nel.dk, tj@...nel.org, scameron@...rdog.cce.hp.com
Subject: Question about REQ_FLUSH and bios with data


So, I'm working on a block driver using the make_request_fn
interface, and have to handle a bio that comes in with
(bi_rw & REQ_FLUSH) set AND data to transfer. 

According to Documentation/block/writeback_cache_control.txt:

	The REQ_FLUSH flag can be OR ed into the r/w flags of a
	bio submitted from the filesystem and will make sure the
	volatile cache of the storage device has been flushed
	before the actual I/O operation is started. 

So I've written code that handles that case, along with the
various error cases I might encounter, I think, which leads
to my question:

How do I get such a bio with a data transfer AND the REQ_FLUSH bit
set to come into the driver?   Just wondering how to test this case.

Thanks,

(I cc'ed Tejun Heo just because his name is in block/blk-flush.c)

-- steve

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