[RFC PATCH 0/9] ublk_drv: add USER_RECOVERY support

ZiyangZhang
2022-08-24T05:48:50Z

ublk_drv is a driver simply passes all blk-mq rqs to ublksrv[1] in
userspace. For each ublk queue, there is one ubq_daemon(pthread).
All ubq_daemons share the same process which opens /dev/ublkcX.
The ubq_daemon code infinitely loops on io_uring_enter() to
send/receive io_uring cmds which pass information of blk-mq
rqs.

Now, if one ubq_daemon(pthread) or the process crashes, ublk_drv
must abort the dying ubq, stop the device and release everything.
This is not a good choice in practice because users do not expect
aborted requests, I/O errors and a released device. They may want
a recovery machenism so that no requests are aborted and no I/O
error occurs. Anyway, users just want everything works as uaual.

This RFC patchset implements USER_RECOVERY support. If the process
crashes, we allow ublksrv to provide new process and ubq_daemons.
We do not support single ubq_daemon(pthread) recovery because a
pthread rarely crashes.

Recovery feature is quite useful for products do not expect to
return any I/O error to frontend users. In detail, we support
this scenario:
(1) The /dev/ublkc0 is opened by process 0;
(2) Fio is running on /dev/ublkb0 exposed by ublk_drv and all
rqs are handled by process 0.
(3) Process 0 suddenly crashes(e.g. segfault);
(4) Fio is still running and submit IOs(but these IOs cannot
complete now)
(5) User recovers with process 1 and attach it to /dev/ublkc0
(6) All rqs are handled by process 1 now and IOs can be
completed now.

Note: The backend must tolerate double-write because we re-issue
a rq sent to the old(dying) process before. We allow users to
choose whether re-issue these rqs or not, please see patch 7 for
more detail.

We provide a sample script here to simulate the above steps:

***************************script***************************
LOOPS=10

__ublk_get_pid() {
pid=`./ublk list -n 0 | grep "pid" | awk '{print $7}'`
echo $pid
}

ublk_recover_kill()
{
for CNT in `seq $LOOPS`; do
dmesg -C
pid=`__ublk_get_pid`
echo -e "*** kill $pid now ***"
kill -9 $pid
sleep 2
echo -e "*** recover now ***"
./ublk recover -n 0
sleep 4
done
}

ublk_test()
{
dmesg -C
echo -e "*** add ublk device ***"
./ublk add -t null -d 4 -i 1
sleep 2
echo -e "*** start fio ***"
fio --bs=4k \
--filename=/dev/ublkb0 \
--runtime=100s \
--rw=read &
sleep 4
ublk_recover_kill
wait
echo -e "*** delete ublk device ***"
./ublk del -n 0
}

for CNT in `seq 4`; do
modprobe -rv ublk_drv
modprobe ublk_drv
echo -e "************ round $CNT ************"
ublk_test
sleep 5
done
***************************script***************************

You may run it with our modified ublksrv[3] which supports
recovey feature. No I/O error occurs and you can verify it
by typing
$ perf-tools/bin/tpoint block:block_rq_error

The basic idea of USER_RECOVERY is quite straightfoward:

(1) release/free everything belongs to the dying process.

Note: Since ublk_drv does save information about user process,
this work is important because we don't expect any resource
lekage. Particularly, ioucmds from the dying ubq_daemons
need to be completed(freed). Current ublk_drv code cannot satisfy
our need while considering USER_RECOVERY. So we refactor some code
shown in patch 1-5 to gracefully free all ioucmds.

(2) init ublk queues including requeuing/aborting rqs.

(3) allow new ubq_daemons issue FETCH_REQ.

Here is steps to reocver:

(1) For a user, after a process crash(how he detect a crash is not related
to this patchset), he sends START_USER_RECOVERY ctrl-cmd to
/dev/ublk-control with a dev_id X (such as 3 for /dev/ublkc3).

(2) Then ublk_drv should perpare for a new process to attach /dev/ublkcX.
We have described this before. The driver must quiesce the request
queue to ban any incoming ublk_queue_rq().

(3) Then, user should start a new process and ubq_daemons(pthreads) and
send FETCH_REQ by io_uring_enter() to make all ubqs be ready. The
user must correctly setup queues, flags and so on(how to persist
ublksrv information is not related to this patchset).

(4) The user sends END_USER_RECOVERY ctrl-cmd to /dev/ublk-control with a
dev_id X.

(5) ublk_drv waits for all ubq_daemons getting ready. Then it unquiesces
request queue and new rqs are allowed.

After applying refactor patches(patch 1-5), with current ublksrv[1], all
tests[2] passes. Note that refactor patches DO NOT involve any recovery
feature.

After applying all patches(patch 1-9), you should use ublksrv[3] and
tests[4] provided by us. We add 2 additional tests to verify that
recovery feature works. our code will be PR-ed to Ming's repo soon.

[1] https://github.com/ming1/ubdsrv
[2] https://github.com/ming1/ubdsrv/tree/master/tests
[3] https://github.com/old-memories/ubdsrv/tree/recovery-v1
[4] https://github.com/old-memories/ubdsrv/tree/recovery-v1/tests/generic

ZiyangZhang (9):
ublk_drv: check 'current' instead of 'ubq_daemon'
ublk_drv: refactor ublk_cancel_queue()
ublk_drv: add a helper to get ioucmd from pdu
ublk_drv: refactor __ublk_rq_task_work() and aborting machenism
ublk_drv: refactor ublk_stop_dev()
ublk_drv: add pr_devel() to prepare for recovery feature
ublk_drv: define macros for recovery feature and check them
ublk_drv: add START_USER_RECOVERY and END_USER_RECOVERY support
ublk_drv: do not schedule monitor_work with recovery feature enabled

drivers/block/ublk_drv.c | 517 ++++++++++++++++++++++++++--------
include/uapi/linux/ublk_cmd.h | 7 +
2 files changed, 408 insertions(+), 116 deletions(-)

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2.27.0