Re: [PATCH 5.4 26/34] dm verity: set DM_TARGET_IMMUTABLE feature flag

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Jun 10 2022 at  1:15P -0400,
Greg KH <gregkh@linuxfoundation.org> wrote:

> On Fri, Jun 10, 2022 at 04:22:00AM +0000, Oleksandr Tymoshenko wrote:
> > I believe this commit introduced a regression in dm verity on systems
> > where data device is an NVME one. Loading table fails with the
> > following diagnostics:
> > 
> > device-mapper: table: table load rejected: including non-request-stackable devices
> > 
> > The same kernel works with the same data drive on the SCSI interface.
> > NVME-backed dm verity works with just this commit reverted.
> > 
> > I believe the presence of the immutable partition is used as an indicator
> > of special case NVME configuration and if the data device's name starts
> > with "nvme" the code tries to switch the target type to
> > DM_TYPE_NVME_BIO_BASED (drivers/md/dm-table.c lines 1003-1010).
> > 
> > The special NVME optimization case was removed in
> > 5.10 by commit 9c37de297f6590937f95a28bec1b7ac68a38618f, so only 5.4 is
> > affected.
> > 
> 
> Why wouldn't 4.9, 4.14, and 4.19 also be affected here?  Should I also
> just queue up 9c37de297f65 ("dm: remove special-casing of bio-based
> immutable singleton target on NVMe") to those older kernels?  If so,
> have you tested this and verified that it worked?

Sorry for the unforeseen stable@ troubles here!

In general we'd be fine to apply commit 9c37de297f65 but to do it
properly would require also making sure commits that remove
"DM_TYPE_NVME_BIO_BASED", like 8d47e65948dd ("dm mpath: remove
unnecessary NVMe branching in favor of scsi_dh checks") are applied --
basically any lingering references to DM_TYPE_NVME_BIO_BASED need to
be removed.

The commit header for 8d47e65948dd documents what
DM_TYPE_NVME_BIO_BASED was used for.. it was dm-mpath specific and
"nvme" mode really never got used by any userspace that I'm aware of.

Sadly I currently don't have the time to do this backport for all N
stable kernels... :(

But if that backport gets out of control: A simpler, albeit stable@
unicorn, way to resolve this is to simply revert 9c37de297f65 and make
it so that DM-mpath and DM core just used bio-based if "nvme" is
requested by dm-mpath, so also in drivers/md/dm-mpath.c e.g.:

@@ -1091,8 +1088,6 @@ static int parse_features(struct dm_arg_set *as, struct multipath *m)

                        if (!strcasecmp(queue_mode_name, "bio"))
                                m->queue_mode = DM_TYPE_BIO_BASED;
			else if (!strcasecmp(queue_mode_name, "nvme"))
-                               m->queue_mode = DM_TYPE_NVME_BIO_BASED;
+                               m->queue_mode = DM_TYPE_BIO_BASED;
                        else if (!strcasecmp(queue_mode_name, "rq"))
                                m->queue_mode = DM_TYPE_REQUEST_BASED;
                        else if (!strcasecmp(queue_mode_name, "mq"))

Mike