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Date: Mon, 31 Mar 2025 08:36:42 -0600
From: Keith Busch <kbusch@kernel.org>
To: Kanchan Joshi <joshi.k@samsung.com>
Cc: Caleb Sander Mateos <csander@purestorage.com>,
	Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
	Sagi Grimberg <sagi@grimberg.me>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Chaitanya Kulkarni <kch@nvidia.com>, linux-nvme@lists.infradead.org,
	io-uring@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 3/3] nvme/ioctl: move fixed buffer lookup to
 nvme_uring_cmd_io()
Message-ID: <Z-qoevl5Jaf7WFsQ@kbusch-mbp.dhcp.thefacebook.com>
References: <20250328154647.2590171-1-csander@purestorage.com>
 <CGME20250328155548epcas5p2368eb1a59883b133a9baf4ac39d6bac6@epcas5p2.samsung.com>
 <20250328154647.2590171-4-csander@purestorage.com>
 <48b9a876-0e3b-4c89-9aa3-b48f502868c3@samsung.com>
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In-Reply-To: <48b9a876-0e3b-4c89-9aa3-b48f502868c3@samsung.com>

On Mon, Mar 31, 2025 at 12:16:58PM +0530, Kanchan Joshi wrote:
> On 3/28/2025 9:16 PM, Caleb Sander Mateos wrote:
> > For NVMe passthru operations with fixed buffers, the fixed buffer lookup
> > happens in io_uring_cmd_import_fixed(). But nvme_uring_cmd_io() can
> > return -EAGAIN first from nvme_alloc_user_request() if all tags in the
> > tag set are in use. This ordering difference is observable when using
> > UBLK_U_IO_{,UN}REGISTER_IO_BUF SQEs to modify the fixed buffer table. If
> > the NVMe passthru operation is followed by UBLK_U_IO_UNREGISTER_IO_BUF
> > to unregister the fixed buffer and the NVMe passthru goes async, the
> > fixed buffer lookup will fail because it happens after the unregister.
> 
> while the patch looks fine, I wonder what setup is required to 
> trigger/test this. Given that io_uring NVMe passthru is on the char 
> device node, and ublk does not take char device as the backing file. 
> Care to explain?

Not sure I understand the question. A ublk daemon can use anything it
wants on the backend. Are you just referring to the public ublksrv
implementation? That's not used here, if that's what you mean.