[Top-posting but please don't...] I've staged this fix for 6.3 inclusion and marked it for stable@: https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.3&id=0ca44fcef241768fd25ee763b3d203b9852f269b Ming, I also staged this similar fix (not reasoned through scenario where it'd actually occur that dm_wq_requeue_work would loop endlessly but its good practice to include cond_resched() in such a workqueue while loop): https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-6.3&id=f77692d65d54665d81815349cc727baa85e8b71d Thanks, Mike On Thu, Feb 16 2023 at 3:30P -0500, Pingfan Liu <piliu@xxxxxxxxxx> wrote: > Hi Ming, > > Thank you for looking into this. > > let me loop in Alasdair, Mike and Zdenek for further comment on LVM stuff > > > Thanks, > > Pingfan > > On Thu, Feb 16, 2023 at 8:08 AM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > On Wed, Feb 15, 2023 at 07:23:40PM +0800, Pingfan Liu wrote: > > > Hi guys, > > > > > > I encountered a hang issue on a s390x system. The tested kernel is > > > not preemptible and booting with "nr_cpus=1" > > > > > > The test steps: > > > umount /home > > > lvremove /dev/rhel_s390x-kvm-011/home > > > ## uncomment "snapshot_autoextend_threshold = 70" and > > > "snapshot_autoextend_percent = 20" in /etc/lvm/lvm.conf > > > > > > systemctl enable lvm2-monitor.service > > > systemctl start lvm2-monitor.service > > > > > > lvremove -y rhel_s390x-kvm-011/thinp > > > lvcreate -L 10M -T rhel_s390x-kvm-011/thinp > > > lvcreate -V 400M -T rhel_s390x-kvm-011/thinp -n src > > > mkfs.ext4 /dev/rhel_s390x-kvm-011/src > > > mount /dev/rhel_s390x-kvm-011/src /mnt > > > for((i=0;i<4;i++)); do dd if=/dev/zero of=/mnt/test$i.img > > > bs=100M count=1; done > > > > > > And the system hangs with the console log [1] > > > > > > The related kernel config > > > > > > CONFIG_PREEMPT_NONE_BUILD=y > > > CONFIG_PREEMPT_NONE=y > > > CONFIG_PREEMPT_COUNT=y > > > CONFIG_SCHED_CORE=y > > > > > > It turns out that when hanging, the kernel is stuck in the dead-loop > > > in the function dm_wq_work() > > > while (!test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) { > > > spin_lock_irq(&md->deferred_lock); > > > bio = bio_list_pop(&md->deferred); > > > spin_unlock_irq(&md->deferred_lock); > > > > > > if (!bio) > > > break; > > > thread_cpu = smp_processor_id(); > > > submit_bio_noacct(bio); > > > } > > > where dm_wq_work()->__submit_bio_noacct()->...->dm_handle_requeue() > > > keeps generating new bio, and the condition "if (!bio)" can not be > > > meet. > > > > > > > > > After applying the following patch, the issue is gone. > > > > > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c > > > index e1ea3a7bd9d9..95c9cb07a42f 100644 > > > --- a/drivers/md/dm.c > > > +++ b/drivers/md/dm.c > > > @@ -2567,6 +2567,7 @@ static void dm_wq_work(struct work_struct *work) > > > break; > > > > > > submit_bio_noacct(bio); > > > + cond_resched(); > > > } > > > } > > > > > > But I think it is not a proper solution. And without this patch, if > > > removing nr_cpus=1 (the system has two cpus), the issue can not be > > > triggered. That says when more than one cpu, the above loop can exit > > > by the condition "if (!bio)" > > > > > > Any ideas? > > > > I think the patch is correct. > > > > For kernel built without CONFIG_PREEMPT, in case of single cpu core, > > if the dm target(such as dm-thin) needs another wq or kthread for > > handling IO, then dm target side is blocked because dm_wq_work() > > holds the single cpu, sooner or later, dm target may have not > > resource to handle new io from dm core and returns REQUEUE. > > > > Then dm_wq_work becomes one dead loop. > > > > > > Thanks, > > Ming > > > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
