On 1/27/20 12:32 PM, Mike Snitzer wrote: > On Thu, Jan 23 2020 at 1:52pm -0500, > Jens Axboe <axboe@xxxxxxxxx> wrote: > >> 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. > > I just staged the following DM fix: > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.6&id=28a101d6b344f5a38d482a686d18b1205bc92333 I like that a lot more than the NULL check in the core. -- Jens Axboe
