On Thu, Jan 23 2020 at 4:17am -0500, Stefan Bader <stefan.bader@xxxxxxxxxxxxx> 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
