Date:   Tue, 5 Jul 2022 16:12:19 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Ziyang Zhang <ZiyangZhang@...ux.alibaba.com>
Cc:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Harris James R <james.r.harris@...el.com>,
        linux-kernel@...r.kernel.org, io-uring@...r.kernel.org,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, ming.lei@...hat.com
Subject: Re: [PATCH V3 1/1] ublk: add io_uring based userspace block driver

On Tue, Jul 05, 2022 at 12:16:07PM +0800, Ziyang Zhang wrote:
> On 2022/7/5 06:10, Gabriel Krisman Bertazi wrote:
> > Ming Lei <ming.lei@...hat.com> writes:
> > 
> >> This is the driver part of userspace block driver(ublk driver), the other
> >> part is userspace daemon part(ublksrv)[1].
> >>
> >> The two parts communicate by io_uring's IORING_OP_URING_CMD with one
> >> shared cmd buffer for storing io command, and the buffer is read only for
> >> ublksrv, each io command is indexed by io request tag directly, and
> >> is written by ublk driver.
> >>
> >> For example, when one READ io request is submitted to ublk block driver, ublk
> >> driver stores the io command into cmd buffer first, then completes one
> >> IORING_OP_URING_CMD for notifying ublksrv, and the URING_CMD is issued to
> >> ublk driver beforehand by ublksrv for getting notification of any new io request,
> >> and each URING_CMD is associated with one io request by tag.
> >>
> >> After ublksrv gets the io command, it translates and handles the ublk io
> >> request, such as, for the ublk-loop target, ublksrv translates the request
> >> into same request on another file or disk, like the kernel loop block
> >> driver. In ublksrv's implementation, the io is still handled by io_uring,
> >> and share same ring with IORING_OP_URING_CMD command. When the target io
> >> request is done, the same IORING_OP_URING_CMD is issued to ublk driver for
> >> both committing io request result and getting future notification of new
> >> io request.
> >>
> >> Another thing done by ublk driver is to copy data between kernel io
> >> request and ublksrv's io buffer:
> >>
> >> 1) before ubsrv handles WRITE request, copy the request's data into
> >> ublksrv's userspace io buffer, so that ublksrv can handle the write
> >> request
> >>
> >> 2) after ubsrv handles READ request, copy ublksrv's userspace io buffer
> >> into this READ request, then ublk driver can complete the READ request
> >>
> >> Zero copy may be switched if mm is ready to support it.
> >>
> >> ublk driver doesn't handle any logic of the specific user space driver,
> >> so it should be small/simple enough.
> >>
> >> [1] ublksrv
> >>
> >> https://github.com/ming1/ubdsrv
> >>
> >> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> > 
> > Hi Ming,
> > 
> > A few comments inline:
> > 
> > 
> >> +#define UBLK_MINORS		(1U << MINORBITS)
> >> +
> >> +struct ublk_rq_data {
> >> +	struct callback_head work;
> >> +};
> >> +
> >> +/* io cmd is active: sqe cmd is received, and its cqe isn't done */
> >> +#define UBLK_IO_FLAG_ACTIVE	0x01
> >> +
> >> +/*
> >> + * FETCH io cmd is completed via cqe, and the io cmd is being handled by
> >> + * ublksrv, and not committed yet
> >> + */
> >> +#define UBLK_IO_FLAG_OWNED_BY_SRV 0x02
> >> +
> > 
> > Minor nit: I wonder if the IO life cycle isn't better represented as a
> > state machine than flags:
> > 
> > enum {
> >    UBLK_IO_FREE,
> >    UBLK_IO_QUEUED
> >    UBLK_IO_OWNED_BY_SRV
> >    UBLK_IO_COMPLETED,
> >    UBLK_IO_ABORTED,
> > }
> > 
> > Since currently, IO_FLAG_ACTIVE and IO_OWNED_BY_SRV should (almost) be
> > mutually exclusive.
> > 
> > 
> >> +
> >> +static int ublk_ctrl_stop_dev(struct ublk_device *ub)
> >> +{
> >> +	ublk_stop_dev(ub);
> >> +	cancel_work_sync(&ub->stop_work);
> >> +	return 0;
> >> +}
> >> +
> >> +static inline bool ublk_queue_ready(struct ublk_queue *ubq)
> >> +{
> >> +	return ubq->nr_io_ready == ubq->q_depth;
> >> +}
> >> +
> >> +/* device can only be started after all IOs are ready */
> >> +static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
> >> +{
> >> +	mutex_lock(&ub->mutex);
> >> +	ubq->nr_io_ready++;
> > 
> > I think this is still problematic for the case where a FETCH_IO is sent
> > from a different thread than the one originally set in ubq_daemon
> > (i.e. a userspace bug).  Since ubq_daemon is used to decide what task
> > context will do the data copy, If an IO_FETCH_RQ is sent to the same queue
> > from two threads, the data copy can happen in the context of the wrong
> > task.  I'd suggest something like the check below at the beginning of
> > mark_io_ready and a similar on for IO_COMMIT_AND_FETCH_RQ
> > 
> > 	mutex_lock(&ub->mutex);
> >         if (ub->ubq_daemon && ub->ubq_daemon != current) {
> >            mutex_unlock(&ub->mutex);
> >            return -EINVAL;
> >         }
> > 	ubq->nr_io_ready++;
> >         ...
> >> +	if (ublk_queue_ready(ubq)) {
> >> +		ubq->ubq_daemon = current;
> >> +		get_task_struct(ubq->ubq_daemon);
> >> +		ub->nr_queues_ready++;
> >> +	}
> >> +	if (ub->nr_queues_ready == ub->dev_info.nr_hw_queues)
> >> +		complete_all(&ub->completion);
> >> +	mutex_unlock(&ub->mutex);
> >> +}
> >> +
> >> +static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
> >> +{
> >> +	struct ublksrv_io_cmd *ub_cmd = (struct ublksrv_io_cmd *)cmd->cmd;
> >> +	struct ublk_device *ub = cmd->file->private_data;
> >> +	struct ublk_queue *ubq;
> >> +	struct ublk_io *io;
> >> +	u32 cmd_op = cmd->cmd_op;
> >> +	unsigned tag = ub_cmd->tag;
> >> +	int ret = -EINVAL;
> >> +
> >> +	pr_devel("%s: receieved: cmd op %d queue %d tag %d result %d\n",
> > 
> >                          ^^^
> >                          received
> > 
> > 
> >> +			__func__, cmd->cmd_op, ub_cmd->q_id, tag,
> >> +			ub_cmd->result);
> >> +
> >> +	if (!(issue_flags & IO_URING_F_SQE128))
> >> +		goto out;
> >> +
> >> +	ubq = ublk_get_queue(ub, ub_cmd->q_id);
> >> +	if (!ubq || ub_cmd->q_id != ubq->q_id)
> > 
> > q_id is coming from userspace and is used to access an array inside
> > ublk_get_queue().  I think you need to ensure qid < ub->dev_info.nr_hw_queues
> > before calling ublk_get_queue() to protect from a kernel bad memory
> > access triggered by userspace.
> > 
> >> +		goto out;
> >> +
> >> +	if (WARN_ON_ONCE(tag >= ubq->q_depth))
> > 
> > Userspace shouldn't be able to easily trigger a WARN_ON.
> > 
> >> +		goto out;
> >> +
> >> +	io = &ubq->ios[tag];
> >> +
> >> +	/* there is pending io cmd, something must be wrong */
> >> +	if (io->flags & UBLK_IO_FLAG_ACTIVE) {b
> >> +		ret = -EBUSY;
> >> +		goto out;
> >> +	}
> >> +
> >> +	switch (cmd_op) {
> >> +	case UBLK_IO_FETCH_REQ:
> >> +		/* UBLK_IO_FETCH_REQ is only allowed before queue is setup */
> >> +		if (WARN_ON_ONCE(ublk_queue_ready(ubq))) {
> > 
> > Likewise, this shouldn't trigger a WARN_ON, IMO.
> > 
> >> +			ret = -EBUSY;
> >> +			goto out;
> >> +		}
> >> +		/*
> >> +		 * The io is being handled by server, so COMMIT_RQ is expected
> >> +		 * instead of FETCH_REQ
> >> +		 */
> >> +		if (io->flags & UBLK_IO_FLAG_OWNED_BY_SRV)
> >> +			goto out;
> >> +		/* FETCH_RQ has to provide IO buffer */
> >> +		if (!ub_cmd->addr)
> >> +			goto out;
> >> +		io->cmd = cmd;
> >> +		io->flags |= UBLK_IO_FLAG_ACTIVE;
> >> +		io->addr = ub_cmd->addr;
> >> +
> >> +		ublk_mark_io_ready(ub, ubq);
> >> +		break;
> >> +	case UBLK_IO_COMMIT_AND_FETCH_REQ:
> >> +		/* FETCH_RQ has to provide IO buffer */
> >> +		if (!ub_cmd->addr)
> >> +			goto out;
> >> +		io->addr = ub_cmd->addr;
> >> +		io->flags |= UBLK_IO_FLAG_ACTIVE;
> >> +		fallthrough;
> >> +	case UBLK_IO_COMMIT_REQ:
> >> +		io->cmd = cmd;
> >> +		if (!(io->flags & UBLK_IO_FLAG_OWNED_BY_SRV))
> >> +			goto out;
> >> +		ublk_commit_completion(ub, ub_cmd);
> >> +
> >> +		/* COMMIT_REQ is supposed to not fetch req */
> > 
> > I wonder if we could make it without IO_COMMIT_REQ.  Is it useful to be
> > able to commit without fetching a new request?
> 
> UBLK_IO_COMMIT_REQ is not necessary, IMO. 
> In current version of ubd_drv.c I find UBLK_IO_COMMIT_REQ is sent by ublksrv
> after it gets one UBD_IO_RES_ABORT beacuse ubd_drv wants to abort IOs and let
> the ublk daemon exit.
> 
> We can use UBLK_IO_COMMIT_AND_FETCH_REQ to replace UBLK_IO_COMMIT_REQ.
> The data flow could be:
> 
> 1) UBLK_IO_COMMIT_AND_FETCH_REQ from ublksrv
> 
> 2) ubd_drv receives IO's sqe with UBLK_IO_COMMIT_AND_FETCH_REQ
>    and sets the IO's status to UBLK_IO_QUEUED

Not see where UBLK_IO_QUEUED is? :-)

> 
> 3) ubd_drv wants to abort IOs so it just completes
>    this IO's cqe(UBD_IO_RES_ABORT)
> 
> I successfully removed UBLK_IO_COMMIT_REQ when developing libubd
> although I choose the earliest version of ubd_drv.c(v5.17-ubd-dev)
> which may be a buggy version.

You can verify it with latest ublksrv(libublksrv has been in master)
too by the following patch, then 'make test T=generic' can run
successfully.

diff --git a/lib/ublksrv.c b/lib/ublksrv.c
index c4bb2f4..aee71a0 100644
--- a/lib/ublksrv.c
+++ b/lib/ublksrv.c
@@ -150,7 +150,7 @@ static inline int ublksrv_queue_io_cmd(struct ublksrv_queue *q,
                else
                        cmd_op = UBLK_IO_FETCH_REQ;
        } else if (io->flags & UBLKSRV_NEED_COMMIT_RQ_COMP) {
-                       cmd_op = UBLK_IO_COMMIT_REQ;
+                       cmd_op = UBLK_IO_COMMIT_AND_FETCH_REQ;
        } else {
                syslog(LOG_ERR, "io flags is zero, tag %d\n",
                                (int)cmd->tag);

ublk_cancel_queue() will cancel all these io commands.


Thanks,
Ming

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