[REGRESSION] v5.17-rc1+: FIFREEZE ioctl system call hangs

["Thomas Deutschmann" <whissi@whissi.de>]

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