Question about REQ_FLUSH and bios with data

From: scameron
Date: Fri Aug 09 2013 - 15:10:02 EST

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.


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

-- steve

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