On 27.01.20 20:32, 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 Thanks Mike, yeah this looks like it resolves the problem without adding any impact on the generic I/O path. We certainly had thought about that but felt uncertain whether it would not open other risks. Like something adding requests just before the table load. Could this cause some I/O be handled by one function and the rest by another? And would that really matter? The other thing that was a bit strange but maybe someone else's problem is that mount generated I/O requests to start with. The device size should be 0 still. > > From: Mike Snitzer <snitzer@xxxxxxxxxx> > Date: Mon, 27 Jan 2020 14:07:23 -0500 > Subject: [PATCH] dm: fix potential for q->make_request_fn NULL pointer > > Move blk_queue_make_request() to dm.c:alloc_dev() so that > q->make_request_fn is never NULL during the lifetime of a DM device > (even one that is created without a DM table). > > Otherwise generic_make_request() will crash simply by doing: > dmsetup create -n test > mount /dev/dm-N /mnt > > While at it, move ->congested_data initialization out of > dm.c:alloc_dev() and into the bio-based specific init method. > > Reported-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx> > BugLink: https://bugs.launchpad.net/bugs/1860231 > Fixes: ff36ab34583a ("dm: remove request-based logic from make_request_fn wrapper") > Depends-on: c12c9a3c3860c ("dm: various cleanups to md->queue initialization code") > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> > --- > drivers/md/dm.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c > index e8f9661a10a1..b89f07ee2eff 100644 > --- a/drivers/md/dm.c > +++ b/drivers/md/dm.c > @@ -1859,6 +1859,7 @@ static void dm_init_normal_md_queue(struct mapped_device *md) > /* > * Initialize aspects of queue that aren't relevant for blk-mq > */ > + md->queue->backing_dev_info->congested_data = md; > md->queue->backing_dev_info->congested_fn = dm_any_congested; > } > > @@ -1949,7 +1950,12 @@ static struct mapped_device *alloc_dev(int minor) > if (!md->queue) > goto bad; > md->queue->queuedata = md; > - md->queue->backing_dev_info->congested_data = md; > + /* > + * default to bio-based required ->make_request_fn until DM > + * table is loaded and md->type established. If request-based > + * table is loaded: blk-mq will override accordingly. > + */ > + blk_queue_make_request(md->queue, dm_make_request); > > md->disk = alloc_disk_node(1, md->numa_node_id); > if (!md->disk) > @@ -2264,7 +2270,6 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t) > case DM_TYPE_DAX_BIO_BASED: > case DM_TYPE_NVME_BIO_BASED: > dm_init_normal_md_queue(md); > - blk_queue_make_request(md->queue, dm_make_request); > break; > case DM_TYPE_NONE: > WARN_ON_ONCE(true); >
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