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Subject: Re: [PATCH v4 3/3] nvme/ioctl: move fixed buffer lookup to
nvme_uring_cmd_io()
To: Caleb Sander Mateos <csander@purestorage.com>, Keith Busch
<kbusch@kernel.org>, Jens Axboe <axboe@kernel.dk>, Christoph Hellwig
<hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>, Pavel Begunkov
<asml.silence@gmail.com>
Cc: Chaitanya Kulkarni <kch@nvidia.com>, linux-nvme@lists.infradead.org,
io-uring@vger.kernel.org, linux-kernel@vger.kernel.org
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References: <20250328154647.2590171-1-csander@purestorage.com>
<CGME20250328155548epcas5p2368eb1a59883b133a9baf4ac39d6bac6@epcas5p2.samsung.com>
<20250328154647.2590171-4-csander@purestorage.com>
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?