Message-ID: <Z+sifI6fujsc186S@dev-ushankar.dev.purestorage.com>
Date: Mon, 31 Mar 2025 17:17:16 -0600
From: Uday Shankar <ushankar@...estorage.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Shuah Khan <shuah@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	linux-block@...r.kernel.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] ublk: improve handling of saturated queues when ublk
 server exits

On Thu, Mar 27, 2025 at 09:23:21AM +0800, Ming Lei wrote:
> On Wed, Mar 26, 2025 at 11:54:16AM -0600, Uday Shankar wrote:
> > On Wed, Mar 26, 2025 at 01:38:35PM +0800, Ming Lei wrote:
> > > On Tue, Mar 25, 2025 at 04:19:34PM -0600, Uday Shankar wrote:
> > > > There are currently two ways in which ublk server exit is detected by
> > > > ublk_drv:
> > > > 
> > > > 1. uring_cmd cancellation. If there are any outstanding uring_cmds which
> > > >    have not been completed to the ublk server when it exits, io_uring
> > > >    calls the uring_cmd callback with a special cancellation flag as the
> > > >    issuing task is exiting.
> > > > 2. I/O timeout. This is needed in addition to the above to handle the
> > > >    "saturated queue" case, when all I/Os for a given queue are in the
> > > >    ublk server, and therefore there are no outstanding uring_cmds to
> > > >    cancel when the ublk server exits.
> > > > 
> > > > The second method detects ublk server exit only after a long delay
> > > > (~30s, the default timeout assigned by the block layer). Any
> > > > applications using the ublk device will be left hanging for these 30s
> > > > before seeing an error/knowing anything went wrong. This problem is
> > > > illustrated by running the new test_generic_02 against a ublk_drv which
> > > > doesn't have the fix:
> > > > 
> > > > selftests: ublk: test_generic_02.sh
> > > > dev id is 0
> > > > dd: error writing '/dev/ublkb0': Input/output error
> > > > 1+0 records in
> > > > 0+0 records out
> > > > 0 bytes copied, 30.0611 s, 0.0 kB/s
> > > > DEAD
> > > > dd took 31 seconds to exit (>= 5s tolerance)!
> > > > generic_02 : [FAIL]
> > > > 
> > > > Fix this by instead handling the saturated queue case in the ublk
> > > > character file release callback. This happens during ublk server exit
> > > > and handles the issue much more quickly than an I/O timeout:
> > > 
> > > Another solution is to override default 30sec 'timeout'.
> > 
> > Yes, but that still will introduce unnecessary delays, since it is a
> > polling-based solution (very similar to monitor_work we used to have).
> > Also it will add complexity to the unprivileged case, since that
> > actually cares about timeout and we will have to track the "real"
> > timeout separately.
> > 
> > > 
> > > > 
> > > > selftests: ublk: test_generic_02.sh
> > > > dev id is 0
> > > > dd: error writing '/dev/ublkb0': Input/output error
> > > > 1+0 records in
> > > > 0+0 records out
> > > > 0 bytes copied, 0.0376731 s, 0.0 kB/s
> > > > DEAD
> > > > generic_02 : [PASS]
> > > > 
> > > > Signed-off-by: Uday Shankar <ushankar@...estorage.com>
> > > > ---
> > > >  drivers/block/ublk_drv.c                        | 40 +++++++++++------------
> > > >  tools/testing/selftests/ublk/Makefile           |  1 +
> > > >  tools/testing/selftests/ublk/kublk.c            |  3 ++
> > > >  tools/testing/selftests/ublk/kublk.h            |  3 ++
> > > >  tools/testing/selftests/ublk/null.c             |  4 +++
> > > >  tools/testing/selftests/ublk/test_generic_02.sh | 43 +++++++++++++++++++++++++
> > > >  6 files changed, 72 insertions(+), 22 deletions(-)
> > > > 
> > > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > > index c060da409ed8a888b7e414c9065efd2cbd6d57d7..1816b2cac01056dc9d01455759594af43c5f78d6 100644
> > > > --- a/drivers/block/ublk_drv.c
> > > > +++ b/drivers/block/ublk_drv.c
> > > > @@ -1247,8 +1247,6 @@ static void ublk_queue_cmd(struct ublk_queue *ubq, struct request *rq)
> > > >  static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > > >  {
> > > >  	struct ublk_queue *ubq = rq->mq_hctx->driver_data;
> > > > -	unsigned int nr_inflight = 0;
> > > > -	int i;
> > > >  
> > > >  	if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) {
> > > >  		if (!ubq->timeout) {
> > > > @@ -1259,26 +1257,6 @@ static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > > >  		return BLK_EH_DONE;
> > > >  	}
> > > >  
> > > > -	if (!ubq_daemon_is_dying(ubq))
> > > > -		return BLK_EH_RESET_TIMER;
> > > > -
> > > > -	for (i = 0; i < ubq->q_depth; i++) {
> > > > -		struct ublk_io *io = &ubq->ios[i];
> > > > -
> > > > -		if (!(io->flags & UBLK_IO_FLAG_ACTIVE))
> > > > -			nr_inflight++;
> > > > -	}
> > > > -
> > > > -	/* cancelable uring_cmd can't help us if all commands are in-flight */
> > > > -	if (nr_inflight == ubq->q_depth) {
> > > > -		struct ublk_device *ub = ubq->dev;
> > > > -
> > > > -		if (ublk_abort_requests(ub, ubq)) {
> > > > -			schedule_work(&ub->nosrv_work);
> > > > -		}
> > > > -		return BLK_EH_DONE;
> > > > -	}
> > > > -
> > > >  	return BLK_EH_RESET_TIMER;
> > > >  }
> > > >  
> > > > @@ -1351,6 +1329,24 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
> > > >  static int ublk_ch_release(struct inode *inode, struct file *filp)
> > > >  {
> > > >  	struct ublk_device *ub = filp->private_data;
> > > > +	bool need_schedule = false;
> > > > +	int i;
> > > > +
> > > > +	/*
> > > > +	 * Error out any requests outstanding to the ublk server. This
> > > > +	 * may have happened already (via uring_cmd cancellation), in
> > > > +	 * which case it is not harmful to repeat. But uring_cmd
> > > > +	 * cancellation does not handle queues which are fully saturated
> > > > +	 * (all requests in ublk server), because from the kernel's POV,
> > > > +	 * there are no outstanding uring_cmds to cancel. This code
> > > > +	 * handles such queues.
> > > > +	 */
> > > > +
> > > > +	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > > > +		need_schedule |= ublk_abort_requests(ub, ublk_get_queue(ub, i));
> > > > +
> > > > +	if (need_schedule)
> > > > +		schedule_work(&ub->nosrv_work);
> > > 
> > > ublk_abort_requests() should be called only in case of queue dying,
> > > since ublk server may open & close the char device multiple times.
> > 
> > Sure that is technically possible, however is any real ublk server doing
> > this? Seems like a strange thing to do, and seems reasonable for the
> > driver to transition the device to the nosrv state (dead or recovery,
> > depending on flags) when the char device is closed, since in this case,
> > no one can be handling I/O anymore.
> 
> ublk server should be free to open & close the char device multiple times,
> but you patch limits ublk server to open & close the char device just once.
> 
> The limit looks too strong...

Tying a userspace daemon lifetime to the file seems to also be done in
fuse, which is very similar to ublk_drv. See e.g. the description here:

https://lore.kernel.org/lkml/20240524064030.4944-1-jefflexu@linux.alibaba.com/T/

This seems required to support certain workflows, e.g. using an fdstore
with ublk devices. While we still keep task references in ublk_drv,
these workflows will be broken.

I am not familiar with fuse so I don't know for sure, but it sounds like
if the file is closed after some setup is performed, then the connection
is aborted. The analogy in ublk might be - if the file is closed while
the device is LIVE, then we transition to the nosrv state. Otherwise
nothing happens and the file can be reopened freely. This will allow
libublksrv to continue working as it opens/closes the fd early to
determine if it is accessible. Does that sound any better?

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