On Thu, Jan 23 2020 at 5:35am -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > 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. SO I had a look and there isn't anything easier than adding the proposed NULL check in generic_make_request_checks(). Given the many conditionals in that function.. what's one more? ;) I looked at marking the queue frozen to prevent IO via blk_queue_enter()'s existing cheeck -- but that quickly felt like an abuse, especially in that there isn't a queue unfreeze for bio-based. Jens, I'll defer to you to judge this patch further. If you're OK with it: cool. If not, I'm open to suggestions for how to proceed.
