Re: [PATCH v2 1/4] blk-flush: flush_rq should inherit first_rq's cmd_flags

On Tue, Aug 01, 2023 at 01:04:32PM +0200, Christoph Hellwig wrote:

On Mon, Jul 31, 2023 at 06:28:01PM +0200, Hannes Reinecke wrote:

quoted

The flush machinery is sending flushes before and/or after the original 
request (preflush/postflush). For blocked transports (ie during FC RSCN 
handling) the transport will error out commands depending on the FAILFAST 
setting. If FAILFAST is set the SCSI layer gets an STS_TRANSPORT error 
(causing the I/O to be retried), but STS_ERROR if not set (causing I/O to 
failed).

So if the FAILFAST setting is _not_ aligned between flush_rq and the 
original we'll get an error on the flush rq and a retry on the original rq, 
causing the entire command to fail.

I guess we need to align them.

But you can't, because multiple pre/postflushes are coalesced into a
single outstanding flush request.  They can and will not match quite
commonly.

And if you mean the REQ_FAILFAST_TRANSPORT added by dm - this will
never even see the flush state machine, as that is run in dm-mpath
which then inserts the fully built flush request into the lower request
queue.  At least for request based multipath, bio could hit it.