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

[Ming Lei]

On Mon, Aug 29, 2022 at 12:00:49PM +0800, Ziyang Zhang wrote:
> On 2022/8/29 10:08, Ming Lei wrote:
> > On Wed, Aug 24, 2022 at 01:47:35PM +0800, ZiyangZhang wrote:
> >> 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.
> > 
> > I understand the requirement is that both /dev/ublkbN and /dev/ublkcN
> > won't be deleted & re-added from user viewpoint after user recovery,
> > so the device context won't be lost.
> 
> Yes, after the 'process' is killed or crashed(such as segmentation fault)
> both /dev/ublkb0 and /dev/ublkc0 is not deleted.
> 
> > 
> >>
> >> 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.
> > 
> > That looks one very ideal requirement. To be honest, no any block driver
> > can guarantee that 100%, so it is just one soft requirement?
> > 
> > Cause memory allocation may fail, network may be disconnected,
> > re-creating pthread or process may fail too, ...
> 
> Yes, I know there are many other problem which may cause a failure.
> 
> The recovery mechanism only guarantees that rqs sent to ublksrv
> before crash are not aborted. Instead, ublk_drv re-issues the request
> itself and fio does not konw about it. Of course the backend must
> tolerate a double-write/read.

My comment is for 'do not expect to return any I/O error to frontend users',
and I still think it is just one soft requirement, and no one can
guarantee there isn't any error for frontend users really.

> 
> > 
> >> 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
> > 
> > The current behavior is that /dev/ublkb* is removed after device is
> > aborted because ubq daemon is killed.
> > 
> > What if 'ublk recover' command isn't sent? So the current behavior
> > without recovery is changed? Or just changed with this feature enabled?
> 
> No, I do not change the default behavior. You can verify this by running
> generic/002 and generic/003. These tests passes with either recovery enabled
> or disabled.
> 
> 
> (1) With recovery disabled, the monitor_work scheduled periodically or
>     STOP_DEV ctrl-cmd issued manually can cleanup everything and remove the
>     gendisk.
> 
> (2)With recovery enabled, the monitor_work is not scheduled anymore, see
>    patch 9. So after a crash，all resources are still in kernel.
>    Then, there are two options for a user:
>     (a) You don't want to recover it, so just send STOP_DEV ctrl-cmd. This will
>         schedule monitor_work once and cleanup everything. Please see patch 5.

But what if people sends nothing and starts to reboot, then hang forever without
monitor_work involved.

>     (b) You want to recover it, so just send START_RECOVERY ctrl-cmd. Then you
>         HAVE TO start a new process and send END_RECOVERY.
> 
> Note: Actually I am thinking what if a user has sent START_RECOVERY but he fails
> to start a new process. I have a rough idea: just abort all rqs after we unqiuesce
> the request queue. But that is not included in this RFC patchset because I
> want to make it simpler. Maybe we can consider it later?

It is pretty easy to fail all in-queue requests when user recovery
can't move on.

> 
> > 
> > BTW, I do not mean the change isn't reasonable, but suggest to document
> > the user visible change, so it can get reviewed from either user
> > viewpoint and technical point.
> > 
> >> 		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
> > 
> > I'd suggest to describe crash detector a bit at least, as one whole use case,
> > crash detector should be the input of the use case of user recovery, which is
> > usually one part of use case when modeling software requirement/design.
> 
> This patchset tries to answer only one question: After a process crash, how to
> re-attach the device by another process. So I do not consider other questions
> too much, such as:
> (1) How to detect a crash?
> (2) Is IO hang a crash? Should we kill the process?
> (3) What if a blk-mq rq timeout? Does the process dies? Should we kill the process?

But you have to define what is 'crash', otherwise how can you define
what to be recovered?

So far please just define the crash as the whole daemon being dead
abnormally(without sending stop command) if you don't have better idea.

> 
> I think we can answer them after kernel-support of USER_RECOVERY is available.
> 
> 
> For now I only try to directly kill the process in testcases and manually inject
> a crash in handle_io_async().
> 
> > 
> > Such as, crash is detected after the ubq daemon pthread/process is crashed?
> > Will you consider io hang in the daemon pthread/process? IMO, long-term,
> > the crash detector utility should be part of ublksrv.
> 
> Yes, we should design a crash detector in ublksrv. For IO hang, my idea is that:
> 
> (1) the ublksrv_tgt code should handle it if user runs ublksrv directly.
> 
> (2) the backend should handle it if user only uses libublksrv and embeds it inside
>     the backend code.
> 
> > 
> > We don't implement ublk driver's IO timeout yet, but that implementation may be
> > related with this recovery feature closely, such as, one simple approach is to
> > kill ubq-daemon if we can't move on after retrying several times, then
> > let userspace detect & recovery.
> 
> You mean the ublk_drv can kill the ubq_daemon? I have not consider this case yet...
> 
> BTW, I don't think we should put too much logic(IO hang, detector) in ublk_drv
> because it should only pass-through rqs to userpsace. We should make ublk_drv simple.
> Accept a new daemon and re-attach it to /dev/ublkb0 is what it can do I think.

Actually I was thinking the use case of container-ware ublk device when
ADMIN privilege requirement can be removed, so people can do whatever
they want in ublksrv. But sooner or later, request timeout needs to be
considered for real ublk use case.

thanks,
Ming