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

From: Pavel Machek <pavel@ucw.cz>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	io-uring@vger.kernel.org,
	Gabriel Krisman Bertazi <krisman@collabora.com>,
	ZiyangZhang <ZiyangZhang@linux.alibaba.com>,
	Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Subject: Re: [RFC PATCH] ubd: add io_uring based userspace block driver
Date: Mon, 30 May 2022 09:07:00 +0200	[thread overview]
Message-ID: <20220530070700.GF1363@bug> (raw)
In-Reply-To: <20220509092312.254354-1-ming.lei@redhat.com>

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?

What are restriction on ubdsrv? What happens when it needs to allocate memory, or is
swapped out?

Have mm people seen this?

Best regards,
										Pavel