Re: [RFC PATCH] ubd: add io_uring based userspace block driver

[Gao Xiang]

On Thu, Jun 02, 2022 at 11:19:42AM +0800, Ming Lei wrote:
> Hello Pavel,
> 
> On Mon, May 30, 2022 at 09:07:00AM +0200, Pavel Machek wrote:
> > Hi!
> > 
> > > This is the driver part of userspace block driver(ubd driver), the other
> > > part is userspace daemon part(ubdsrv)[1].
> > 
> > > @@ -0,0 +1,1193 @@
> > > +// SPDX-License-Identifier: GPL-2.0-or-later
> > > +/*
> > > + * Userspace block device - block device which IO is handled from userspace
> > > + *
> > > + * Take full use of io_uring passthrough command for communicating with
> > > + * ubd userspace daemon(ubdsrvd) for handling basic IO request.
> > 
> > > +
> > > +static inline unsigned int ubd_req_build_flags(struct request *req)
> > > +{
> > ...
> > > +	if (req->cmd_flags & REQ_SWAP)
> > > +		flags |= UBD_IO_F_SWAP;
> > > +
> > > +	return flags;
> > > +}
> > 
> > Does it work? How do you guarantee operation will be deadlock-free with swapping and
> > writebacks going on?
> 
> The above is just for providing command flags to user side, so that the
> user side can understand/handle the request better.
> 
> prtrl(PR_SET_IO_FLUSHER) has been merged for avoiding the deadlock.
>

I've pointed out a case before that (I think) PR_SET_IO_FLUSHER doesn't work:
https://lore.kernel.org/all/YhbYOeMUv5+U1XdQ@B-P7TQMD6M-0146.local

I don't think handling writeback in the userspace under the direct reclaim
context is _safe_ honestly. Because userspace program can call any system
call under direct reclaim, which can interconnect to another process context
and wait for it. yet I don't look into ubd implementation.

Thanks,
Gao Xiang