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

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Date:   Tue, 1 Aug 2023 13:06:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Christoph Hellwig <hch@....de>, chengming.zhou@...ux.dev,
        axboe@...nel.dk, ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, zhouchengming@...edance.com
Subject: 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:
> > 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.