On Thu, Jan 23 2020 at  4:17am -0500,
Stefan Bader [off-list ref] wrote:

When device-mapper adapted for multi-queue functionality, they
also re-organized the way the make-request function was set.
Before, this happened when the device-mapper logical device was
created. Now it is done once the mapping table gets loaded the
first time (this also decides whether the block device is request
or bio based).

However in generic_make_request(), the request function gets used
without further checks and this happens if one tries to mount such
a partially set up device.

This can easily be reproduced with the following steps:
 - dmsetup create -n test
 - mount /dev/dm-<#> /mnt

This maybe is something which also should be fixed up in device-
mapper.

I'll look closer at other options.

But given there is already a check for an unset queue
pointer and potentially there could be other drivers which do or
might do the same, it sounds like a good move to add another check
to generic_make_request_checks() and to bail out if the request
function has not been set, yet.

BugLink: https://bugs.launchpad.net/bugs/1860231

From that bug;
"The currently proposed fix introduces no chance of stability
regressions. There is a chance of a very small performance regression
since an additional pointer comparison is performed on each block layer
request but this is unlikely to be noticeable."

This captures my immediate concern: slowing down everyone for this DM
edge-case isn't desirable.

Mike