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 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); -- 2.21.GIT
