Hi, while trying to backup a Dell R7525 system running Debian bookworm/testing using LVM snapshots I noticed that the system will 'freeze' sometimes (not all the times) when creating the snapshot. First I thought this was related to LVM so I created https://listman.redhat.com/archives/linux-lvm/2022-July/026228.html (continued at https://listman.redhat.com/archives/linux-lvm/2022-August/thread.html#26229) Long story short: I was even able to reproduce with fsfreeze, see last strace lines > [...] > 14471 1659449870.984635 openat(AT_FDCWD, "/var/lib/machines", O_RDONLY) = 3 > 14471 1659449870.984658 newfstatat(3, "", {st_mode=S_IFDIR|0700, st_size=4096, ...}, AT_EMPTY_PATH) = 0 > 14471 1659449870.984678 ioctl(3, FIFREEZE so I started to bisect kernel and found the following bad commit: > md: add support for REQ_NOWAIT > > commit 021a24460dc2 ("block: add QUEUE_FLAG_NOWAIT") added support > for checking whether a given bdev supports handling of REQ_NOWAIT or not. > Since then commit 6abc49468eea ("dm: add support for REQ_NOWAIT and enable > it for linear target") added support for REQ_NOWAIT for dm. This uses > a similar approach to incorporate REQ_NOWAIT for md based bios. > > This patch was tested using t/io_uring tool within FIO. A nvme drive > was partitioned into 2 partitions and a simple raid 0 configuration > /dev/md0 was created. > > md0 : active raid0 nvme4n1p1[1] nvme4n1p2[0] > 937423872 blocks super 1.2 512k chunks > > Before patch: > > $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100 > > Running top while the above runs: > > $ ps -eL | grep $(pidof io_uring) > > 38396 38396 pts/2 00:00:00 io_uring > 38396 38397 pts/2 00:00:15 io_uring > 38396 38398 pts/2 00:00:13 iou-wrk-38397 > > We can see iou-wrk-38397 io worker thread created which gets created > when io_uring sees that the underlying device (/dev/md0 in this case) > doesn't support nowait. > > After patch: > > $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100 > > Running top while the above runs: > > $ ps -eL | grep $(pidof io_uring) > > 38341 38341 pts/2 00:10:22 io_uring > 38341 38342 pts/2 00:10:37 io_uring > > After running this patch, we don't see any io worker thread > being created which indicated that io_uring saw that the > underlying device does support nowait. This is the exact behaviour > noticed on a dm device which also supports nowait. > > For all the other raid personalities except raid0, we would need > to train pieces which involves make_request fn in order for them > to correctly handle REQ_NOWAIT. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i d=f51d46d0e7cb5b8494aa534d276a9d8915a2443d After reverting this commit (and follow up commit 0f9650bd838efe5c52f7e5f40c3204ad59f1964d) v5.18.15 and v5.19 worked for me again. At this point I still wonder why I experienced the same problem even after I removed one nvme device from the mdraid array and tested it separately. So maybe there is another nowait/REQ_NOWAIT problem somewhere. During bisect I only tested against the mdraid array. #regzbot introduced: f51d46d0e7cb5b8494aa534d276a9d8915a2443d #regzbot link: https://listman.redhat.com/archives/linux-lvm/2022-July/026228.html #regzbot link: https://listman.redhat.com/archives/linux-lvm/2022-August/thread.html#26229 -- Regards, Thomas
