On 1/23/20 10:28 AM, Mike Snitzer wrote: > 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. > It does kinda suck... The generic_make_request_checks() is a mess, and this doesn't make it any better. Any reason why we can't solve this two step setup in a clean fashion instead of patching around it like this? Feels like a pretty bad hack, tbh. -- Jens Axboe
