Re: [PATCH RFC] io_uring: Pass whole sqe to commands

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 13, 2023 at 09:47:56AM -0700, Breno Leitao wrote:
> Hello Ming,
> 
> On Thu, Apr 13, 2023 at 10:56:49AM +0800, Ming Lei wrote:
> > On Thu, Apr 06, 2023 at 09:57:05AM -0700, Breno Leitao wrote:
> > > Currently uring CMD operation relies on having large SQEs, but future
> > > operations might want to use normal SQE.
> > > 
> > > The io_uring_cmd currently only saves the payload (cmd) part of the SQE,
> > > but, for commands that use normal SQE size, it might be necessary to
> > > access the initial SQE fields outside of the payload/cmd block.  So,
> > > saves the whole SQE other than just the pdu.
> > > 
> > > This changes slighlty how the io_uring_cmd works, since the cmd
> > > structures and callbacks are not opaque to io_uring anymore. I.e, the
> > > callbacks can look at the SQE entries, not only, in the cmd structure.
> > > 
> > > The main advantage is that we don't need to create custom structures for
> > > simple commands.
> > > 
> > > Suggested-by: Pavel Begunkov <asml.silence@gmail.com>
> > > Signed-off-by: Breno Leitao <leitao@debian.org>
> > > ---
> > 
> > ...
> > 
> > > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> > > index 2e4c483075d3..9648134ccae1 100644
> > > --- a/io_uring/uring_cmd.c
> > > +++ b/io_uring/uring_cmd.c
> > > @@ -63,14 +63,15 @@ EXPORT_SYMBOL_GPL(io_uring_cmd_done);
> > >  int io_uring_cmd_prep_async(struct io_kiocb *req)
> > >  {
> > >  	struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
> > > -	size_t cmd_size;
> > > +	size_t size = sizeof(struct io_uring_sqe);
> > >  
> > >  	BUILD_BUG_ON(uring_cmd_pdu_size(0) != 16);
> > >  	BUILD_BUG_ON(uring_cmd_pdu_size(1) != 80);
> > >  
> > > -	cmd_size = uring_cmd_pdu_size(req->ctx->flags & IORING_SETUP_SQE128);
> > > +	if (req->ctx->flags & IORING_SETUP_SQE128)
> > > +		size <<= 1;
> > >  
> > > -	memcpy(req->async_data, ioucmd->cmd, cmd_size);
> > > +	memcpy(req->async_data, ioucmd->sqe, size);
> > 
> > The copy will make some fields of sqe become READ TWICE, and driver may see
> > different sqe field value compared with the one observed in io_init_req().
> 
> This copy only happens if the operation goes to the async path
> (calling io_uring_cmd_prep_async()).  This only happens if
> f_op->uring_cmd() returns -EAGAIN.
> 
>           ret = file->f_op->uring_cmd(ioucmd, issue_flags);
>           if (ret == -EAGAIN) {
>                   if (!req_has_async_data(req)) {
>                           if (io_alloc_async_data(req))
>                                   return -ENOMEM;
>                           io_uring_cmd_prep_async(req);
>                   }
>                   return -EAGAIN;
>           }
> 
> Are you saying that after this copy, the operation is still reading from
> sqe instead of req->async_data?

I meant that the 2nd read is on the sqe copy(req->aync_data), but same
fields can become different between the two READs(first is done on original
SQE during io_init_req(), and second is done on sqe copy in driver).

Will this kind of inconsistency cause trouble for driver? Cause READ
TWICE becomes possible with this patch.

> 
> If you have an example of the two copes flow, that would be great.

Not any example yet, but also not see any access on cmd->sqe(except for cmd_op)
in your patches too.


Thanks,
Ming