Re: [PATCH 2/5] io_uring: add support for IORING_OP_URING_CMD

Darrick J. Wong
2021-01-28T00:40:44Z

On Wed, Jan 27, 2021 at 02:25:38PM -0700, Jens Axboe wrote:
> This is a file private kind of request. io_uring doesn't know what's
> in this command type, it's for the file_operations->uring_cmd()
> handler to deal with.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
> fs/io_uring.c | 59 +++++++++++++++++++++++++++++++++++
> include/linux/io_uring.h | 12 +++++++
> include/uapi/linux/io_uring.h | 1 +
> 3 files changed, 72 insertions(+)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 03748faa5295..55c2714a591e 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -712,6 +712,7 @@ struct io_kiocb {
> struct io_shutdown shutdown;
> struct io_rename rename;
> struct io_unlink unlink;
> + struct io_uring_cmd uring_cmd;
> /* use only after cleaning per-op data, see io_clean_op() */
> struct io_completion compl;
> };
> @@ -805,6 +806,8 @@ struct io_op_def {
> unsigned needs_async_data : 1;
> /* should block plug */
> unsigned plug : 1;
> + /* doesn't support personality */
> + unsigned no_personality : 1;
> /* size of async data needed, if any */
> unsigned short async_size;
> unsigned work_flags;
> @@ -998,6 +1001,11 @@ static const struct io_op_def io_op_defs[] = {
> .work_flags = IO_WQ_WORK_MM | IO_WQ_WORK_FILES |
> IO_WQ_WORK_FS | IO_WQ_WORK_BLKCG,
> },
> + [IORING_OP_URING_CMD] = {
> + .needs_file = 1,
> + .no_personality = 1,
> + .work_flags = IO_WQ_WORK_MM | IO_WQ_WORK_BLKCG,
> + },
> };
>
> enum io_mem_account {
> @@ -3797,6 +3805,47 @@ static int io_unlinkat(struct io_kiocb *req, bool force_nonblock)
> return 0;
> }
>
> +static void io_uring_cmd_done(struct io_uring_cmd *cmd, ssize_t ret)
> +{
> + struct io_kiocb *req = container_of(cmd, struct io_kiocb, uring_cmd);
> +
> + if (ret < 0)
> + req_set_fail_links(req);
> + io_req_complete(req, ret);
> +}
> +
> +static int io_uring_cmd_prep(struct io_kiocb *req,
> + const struct io_uring_sqe *sqe)
> +{
> + struct io_uring_cmd *cmd = &req->uring_cmd;
> +
> + if (!req->file->f_op->uring_cmd)
> + return -EOPNOTSUPP;
> +
> + memcpy(&cmd->pdu, (void *) &sqe->off, sizeof(cmd->pdu));

Hmmm. struct io_uring_pdu is (by my count) 6x uint64_t (==48 bytes) in
size. This starts copying the pdu from byte 8 in struct io_uring_sqe,
and the sqe is 64 bytes in size.

I guess (having not played much with io_uring) that the stuff in the
first eight bytes of the sqe are header info that's common to all
io_uring operations, and hence not passed to io_uring_cmd*.

Assuming that I got that right, that means that the pdu information
doesn't actually go all the way to the end of the sqe, which currently
is just a bunch of padding. Was that intentional, or does this mean
that io_uring_pdu could actually be 8 bytes longer?

Also, I thought io_uring_seq.user_data was supposed to coincide with
io_uring_pdu.reserved? They don't seem to...?

(I could be totally off here, fwiw.)

The reason why I'm counting bytes so stingily is that xfs_scrub issues
millions upon millions of ioctl calls to scrub an XFS. Wouldn't it be
nice if there was a way to submit a single userspace buffer to the
kernel and let it run every scrubber for that fs object in order? I
could cram all that data into the pdu struct ... if it had 56 bytes of
space.

If not, it wouldn't be a big deal to use one of the data[4] fields as a
pointer to a larger struct, but where's the fun in that? :)

Granted I'm programming speculatively in my head, not building an actual
prototype. There are all kinds of other questions I have, like, can a
uring command handler access the task struct or the userspace memory of
the process it was called from? What happens when the user is madly
pounding on ^C while uring commands are running? I should probably
figure out the answers to those questions and maybe even write/crib a
program first...

--D

> + cmd->done = io_uring_cmd_done;
> + return 0;
> +}
> +
> +static int io_uring_cmd(struct io_kiocb *req, bool force_nonblock)
> +{
> + enum io_uring_cmd_flags flags = 0;
> + struct file *file = req->file;
> + int ret;
> +
> + if (force_nonblock)
> + flags |= IO_URING_F_NONBLOCK;
> +
> + ret = file->f_op->uring_cmd(&req->uring_cmd, flags);
> + /* queued async, consumer will call ->done() when complete */
> + if (ret == -EIOCBQUEUED)
> + return 0;
> + else if (ret < 0)
> + req_set_fail_links(req);
> + io_req_complete(req, ret);
> + return 0;
> +}
> +
> static int io_shutdown_prep(struct io_kiocb *req,
> const struct io_uring_sqe *sqe)
> {
> @@ -6093,6 +6142,8 @@ static int io_req_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> return io_renameat_prep(req, sqe);
> case IORING_OP_UNLINKAT:
> return io_unlinkat_prep(req, sqe);
> + case IORING_OP_URING_CMD:
> + return io_uring_cmd_prep(req, sqe);
> }
>
> printk_once(KERN_WARNING "io_uring: unhandled opcode %d\n",
> @@ -6351,6 +6402,9 @@ static int io_issue_sqe(struct io_kiocb *req, bool force_nonblock,
> case IORING_OP_UNLINKAT:
> ret = io_unlinkat(req, force_nonblock);
> break;
> + case IORING_OP_URING_CMD:
> + ret = io_uring_cmd(req, force_nonblock);
> + break;
> default:
> ret = -EINVAL;
> break;
> @@ -6865,6 +6919,9 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
> if (id) {
> struct io_identity *iod;
>
> + if (io_op_defs[req->opcode].no_personality)
> + return -EINVAL;
> +
> iod = idr_find(&ctx->personality_idr, id);
> if (unlikely(!iod))
> return -EINVAL;
> @@ -10260,6 +10317,8 @@ static int __init io_uring_init(void)
> BUILD_BUG_SQE_ELEM(40, __u16, buf_index);
> BUILD_BUG_SQE_ELEM(42, __u16, personality);
> BUILD_BUG_SQE_ELEM(44, __s32, splice_fd_in);
> + BUILD_BUG_ON(offsetof(struct io_uring_sqe, user_data) !=
> + offsetof(struct io_uring_pdu, reserved));
>
> BUILD_BUG_ON(ARRAY_SIZE(io_op_defs) != IORING_OP_LAST);
> BUILD_BUG_ON(__REQ_F_LAST_BIT >= 8 * sizeof(int));
> diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
> index 35b2d845704d..e4e822d86e22 100644
> --- a/include/linux/io_uring.h
> +++ b/include/linux/io_uring.h
> @@ -34,6 +34,18 @@ struct io_uring_task {
> bool sqpoll;
> };
>
> +struct io_uring_pdu {
> + __u64 data[4]; /* available for free use */
> + __u64 reserved; /* can't be used by application! */
> + __u64 data2; /* available or free use */
> +};
> +
> +struct io_uring_cmd {
> + struct file *file;
> + struct io_uring_pdu pdu;
> + void (*done)(struct io_uring_cmd *, ssize_t);
> +};
> +
> #if defined(CONFIG_IO_URING)
> struct sock *io_uring_get_socket(struct file *file);
> void __io_uring_task_cancel(void);
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index ac4e1738a9af..0a0de40a3a5c 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -137,6 +137,7 @@ enum {
> IORING_OP_SHUTDOWN,
> IORING_OP_RENAMEAT,
> IORING_OP_UNLINKAT,
> + IORING_OP_URING_CMD,
>
> /* this goes last, obviously */
> IORING_OP_LAST,
> --
> 2.30.0
>