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Date: Wed, 2 Apr 2025 18:51:11 +0530
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Subject: Re: [PATCH v4 3/3] nvme/ioctl: move fixed buffer lookup to
nvme_uring_cmd_io()
To: Keith Busch <kbusch@kernel.org>
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
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From: Kanchan Joshi <joshi.k@samsung.com>
In-Reply-To: <Z-qoevl5Jaf7WFsQ@kbusch-mbp.dhcp.thefacebook.com>
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References: <20250328154647.2590171-1-csander@purestorage.com>
<CGME20250328155548epcas5p2368eb1a59883b133a9baf4ac39d6bac6@epcas5p2.samsung.com>
<20250328154647.2590171-4-csander@purestorage.com>
<48b9a876-0e3b-4c89-9aa3-b48f502868c3@samsung.com>
<Z-qoevl5Jaf7WFsQ@kbusch-mbp.dhcp.thefacebook.com>
On 3/31/2025 8:06 PM, Keith Busch wrote:
> 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.
got it, I did not think beyond public ublksrv.
The userspace block over nvme char-device sounds interesting.