Re: [PATCH for-next 4/4] nvme-multipath: add multipathing for uring-passthrough commands

[Kanchan Joshi <joshi.k@samsung.com>]

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>>>>>The way I would do this that in nvme_ioucmd_failover_req (or in the
>>>>>retry driven from command retriable failure) I would do the above,
>>>>>requeue it and kick the requeue work, to go over the requeue_list and
>>>>>just execute them again. Not sure why you even need an explicit retry
>>>>>code.
>>>>During retry we need passthrough command. But passthrough command is not
>>>>stable (i.e. valid only during first submission). We can make it stable
>>>>either by:
>>>>(a) allocating in nvme (b) return -EAGAIN to io_uring, and it 
>>>>will do allocate + deferral
>>>>Both add a cost. And since any command can potentially fail, that
>>>>means taking that cost for every IO that we issue on mpath node. Even if
>>>>no failure (initial or subsquent after IO) occcured.
>>>
>>>As mentioned, I think that if a driver consumes a command as queued,
>>>it needs a stable copy for a later reformation of the request for
>>>failover purposes.
>>
>>So what do you propose to make that stable?
>>As I mentioned earlier, stable copy requires allocating/copying in fast
>>path. And for a condition (failover) that may not even occur.
>>I really think currrent solution is much better as it does not try to make
>>it stable. Rather it assembles pieces of passthrough command if retry
>>(which is rare) happens.
>
>Well, I can understand that io_uring_cmd is space constrained, otherwise
>we wouldn't be having this discussion. 

Indeed. If we had space for keeping passthrough command stable for
retry, that would really have simplified the plumbing. Retry logic would
be same as first submission.

>However io_kiocb is less
>constrained, and could be used as a context to hold such a space.
>
>Even if it is undesired to have io_kiocb be passed to uring_cmd(), it
>can still hold a driver specific space paired with a helper to obtain it
>(i.e. something like io_uring_cmd_to_driver_ctx(ioucmd) ). Then if the
>space is pre-allocated it is only a small memory copy for a stable copy
>that would allow a saner failover design.

I am thinking along the same lines, but it's not about few bytes of
space rather we need 80 (72 to be precise). Will think more, but
these 72 bytes really stand tall in front of my optimism.

Do you see anything is possible in nvme-side?
Now also passthrough command (although in a modified form) gets copied
into this preallocated space i.e. nvme_req(req)->cmd. This part -

void nvme_init_request(struct request *req, struct nvme_command *cmd) 
{
	...
	memcpy(nvme_req(req)->cmd, cmd, sizeof(*cmd)); 
}

>>>>So to avoid commmon-path cost, we go about doing nothing (no allocation,
>>>>no deferral) in the outset and choose to recreate the passthrough
>>>>command if failure occured. Hope this explains the purpose of
>>>>nvme_uring_cmd_io_retry?
>>>
>>>I think it does, but I'm still having a hard time with it...
>>>
>>Maybe I am reiterating but few main differences that should help -
>>
>>- io_uring_cmd is at the forefront, and bio/request as secondary
>>objects. Former is persistent object across requeue attempts and the
>>only thing available when we discover the path, while other two are
>>created every time we retry.
>>
>>- Receiving bio from upper layer is a luxury that we do not have for
>>  passthrough. When we receive bio, pages are already mapped and we
>>  do not have to deal with user-specific fields, so there is more
>>  freedom in using arbitrary context (workers etc.). But passthrough
>>  command continues to point to user-space fields/buffers, so we need
>>  that task context.
>>
>>>>
>>>>>>+    req = nvme_alloc_user_request(q, &c, nvme_to_user_ptr(d.addr),
>>>>>>+            d.data_len, nvme_to_user_ptr(d.metadata),
>>>>>>+            d.metadata_len, 0, &meta, d.timeout_ms ?
>>>>>>+            msecs_to_jiffies(d.timeout_ms) : 0,
>>>>>>+            ioucmd->cmd_op == NVME_URING_CMD_IO_VEC, 0, 0);
>>>>>>+    if (IS_ERR(req))
>>>>>>+        return PTR_ERR(req);
>>>>>>+
>>>>>>+    req->end_io = nvme_uring_cmd_end_io;
>>>>>>+    req->end_io_data = ioucmd;
>>>>>>+    pdu->bio = req->bio;
>>>>>>+    pdu->meta = meta;
>>>>>>+    req->cmd_flags |= REQ_NVME_MPATH;
>>>>>>+    blk_execute_rq_nowait(req, false);
>>>>>>+    return -EIOCBQUEUED;
>>>>>>+}
>>>>>>+
>>>>>>+void nvme_ioucmd_mpath_retry(struct io_uring_cmd *ioucmd)
>>>>>>+{
>>>>>>+    struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
>>>>>>+    struct nvme_ns_head *head = container_of(cdev, struct 
>>>>>>nvme_ns_head,
>>>>>>+            cdev);
>>>>>>+    int srcu_idx = srcu_read_lock(&head->srcu);
>>>>>>+    struct nvme_ns *ns = nvme_find_path(head);
>>>>>>+    unsigned int issue_flags = IO_URING_F_SQE128 | IO_URING_F_CQE32 |
>>>>>>+        IO_URING_F_MPATH;
>>>>>>+    struct device *dev = &head->cdev_device;
>>>>>>+
>>>>>>+    if (likely(ns)) {
>>>>>>+        struct nvme_uring_cmd_pdu *pdu = nvme_uring_cmd_pdu(ioucmd);
>>>>>>+        struct request *oreq = pdu->req;
>>>>>>+        int ret;
>>>>>>+
>>>>>>+        if (oreq == NULL) {
>>>>>>+            /*
>>>>>>+             * this was not submitted (to device) earlier. For this
>>>>>>+             * ioucmd->cmd points to persistent memory. Free that
>>>>>>+             * up post submission
>>>>>>+             */
>>>>>>+            const void *cmd = ioucmd->cmd;
>>>>>>+
>>>>>>+            ret = nvme_ns_uring_cmd(ns, ioucmd, issue_flags);
>>>>>>+            kfree(cmd);
>>>>>>+        } else {
>>>>>>+            /*
>>>>>>+             * this was submitted (to device) earlier. Use old
>>>>>>+             * request, bio (if it exists) and nvme-pdu to recreate
>>>>>>+             * the command for the discovered path
>>>>>>+             */
>>>>>>+            ret = nvme_uring_cmd_io_retry(ns, oreq, ioucmd, pdu);
>>>>>
>>>>>Why is this needed? Why is reuse important here? Why not always call
>>>>>nvme_ns_uring_cmd?
>>>>
>>>>Please see the previous explanation.
>>>>If condition is for the case when we made the passthrough command stable
>>>>by allocating beforehand.
>>>>Else is for the case when we avoided taking that cost.
>>>
>>>The current design of the multipath failover code is clean:
>>>1. extract bio(s) from request
>>>2. link in requeue_list
>>>3. schedule requeue_work that,
>>>3.1 takes bios 1-by-1, and submits them again (exactly the same way)
>>
>>It is really clean, and fits really well with bio based entry interface.
>>But as I said earlier, it does not go well with uring-cmd based entry
>>interface, and bunch of of other things which needs to be done
>>differently for generic passthrough command.
>>
>>>I'd like us to try to follow the same design where retry is
>>>literally "do the exact same thing, again". That would eliminate
>>>two submission paths that do the same thing, but slightly different.
>>
>>Exact same thing is possible if we make the common path slow i.e.
>>allocate/copy passthrough command and keep it alive until completion.
>>But that is really not the way to go I suppose.
>
>I'm not sure. With Christoph's response, I'm not sure it is
>universally desired to support failover (in my opinion it should). But
>if we do in fact choose to support it, I think we need a better
>solution. If fast-path allocation is your prime concern, then let's try
>to address that with space pre-allocation.

Sure. I understand the benefit that space pre-allocation will give.

And overall, these are the top three things to iron out: 
- to do (failover) or not to do
- better way to keep the passthrough-cmd stable
- better way to link io_uring_cmd

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