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From: Mike Snitzer <>
To: Martin Wilck <>
Cc:, Daniel Wagner <>,,,
	Paolo Bonzini <>,
	Bart Van Assche <>,
	Christoph Hellwig <>,
	Alasdair G Kergon <>
Subject: Re: [dm-devel] [PATCH v3 0/2] dm: dm_blk_ioctl(): implement failover for SG_IO on dm-multipath
Date: Tue, 15 Jun 2021 13:11:37 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Jun 15 2021 at  6:54P -0400,
Martin Wilck <> wrote:

> Hi Mike,
> On Mo, 2021-06-14 at 11:15 -0400, Mike Snitzer wrote:
> > 
> > This work offers a proof-of-concept but it needs further refinement
> > for sure.
> Thanks for looking into it again. I need some more guidance from your
> part in order to be able to resubmit the set in a form that you
> consider ready for merging.
> > The proposed open-coded SCSI code (in patch 2's drivers/md/dm-
> > scsi_ioctl.c) 
> > is well beyond what I'm willing to take in DM.
> I'm not sure what code you're referring to. Is it the processing of the
> bytes of the SCSI result code? If yes, please note that I had changed
> that to open-coded form in response to Bart's review of my v2
> submission. If it's something else, please point it out to me.
> To minimize the special-casing for this code path, Hannes suggested to
> use a target-specific unprepare_ioctl() callback to to tell the generic
> dm code whether a given ioctl could be retried. The logic that I've put
> into dm-scsi_ioctl.c could then be moved into the unprepare_ioctl()
> callback of dm-mpath. dm_blk_ioctl() would need to check the callback's
> return value and possibly retry the ioctl. Would hat appeal to you?
> >   If this type of
> > functionality is still needed (for kvm's SCSI passthru snafu) then
> > more work is needed to negotiate proper interfaces with the SCSI
> > subsystem (added linux-scsi to cc, odd they weren't engaged on this).
> Not cc'ing linux-scsi was my oversight, sorry about that. 
> But I don't quite understand what interfaces you have in mind. SCSI
> needs to expose the SG_IO interface to dm, which it does; I just needed
> to export sg_io() to get access to the sg_io_hdr fields. The question
> whether a given IO can be retried is decided on the dm (-mpath) layer,
> based on blk_status_t; no additional interface on the SCSI side is
> necessary for that.
> > Does it make sense to extend the SCSI device handler interface to add
> > the required enablement? (I think it'd have to if this line of work
> > is
> > to ultimately get upstream).
> My current code uses the device handler indirectly for activating paths
> during priority group switching, via the dm-mpath prepare_ioctl()
> method and __pg_init_all_paths(). This is what I intended - to use
> exactly the same logic for SG_IO which is used for regular read/write
> IO on the block device. What additional functionality for the device
> handler do you have in mind?
> Regards and thanks,
> Martin

I just replied to patch 2 with detailed suggestions.


dm-devel mailing list

      reply	other threads:[~2021-06-15 17:12 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-11 20:25 [dm-devel] [PATCH v3 0/2] dm: dm_blk_ioctl(): implement failover for SG_IO on dm-multipath mwilck
2021-06-11 20:25 ` [dm-devel] [PATCH v3 1/2] scsi: export __scsi_result_to_blk_status() in scsi_ioctl.c mwilck
2021-06-11 20:25 ` [dm-devel] [PATCH v3 2/2] dm: add CONFIG_DM_MULTIPATH_SG_IO - failover for SG_IO on dm-multipath mwilck
2021-06-15 17:10   ` Mike Snitzer
2021-06-16  9:56     ` Martin Wilck
2021-06-14 15:15 ` [dm-devel] [PATCH v3 0/2] dm: dm_blk_ioctl(): implement " Mike Snitzer
2021-06-15 10:54   ` Martin Wilck
2021-06-15 17:11     ` Mike Snitzer [this message]

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