linux-kernel - Re: [PATCH 3/3] io_uring/uring_cmd: import fixed buffer before going async

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Message-ID: <8338ac70-ed0b-4df5-a052-9ab1dfec9e26@gmail.com>
Date: Fri, 21 Mar 2025 20:35:24 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Caleb Sander Mateos <csander@...estorage.com>,
 Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>,
 Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>
Cc: Xinyu Zhang <xizhang@...estorage.com>, io-uring@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH 3/3] io_uring/uring_cmd: import fixed buffer before going
 async

On 3/21/25 18:48, Caleb Sander Mateos wrote:
> For uring_cmd operations with fixed buffers, the fixed buffer lookup
> happens in io_uring_cmd_import_fixed(), called from the ->uring_cmd()
> implementation. A ->uring_cmd() implementation could return -EAGAIN on
> the initial issue for any reason before io_uring_cmd_import_fixed().
> For example, nvme_uring_cmd_io() calls nvme_alloc_user_request() first,
> which can return -EAGAIN if all tags in the tag set are in use.

That's up to command when it resolves the buffer, you can just
move the call to io_import_reg_buf() earlier in nvme cmd code
and not working it around at the io_uring side.

In general, it's a step back, it just got cleaned up from the
mess where node resolution and buffer imports were separate
steps and duplicate by every single request type that used it.

> This ordering difference is observable when using
> UBLK_U_IO_{,UN}REGISTER_IO_BUF SQEs to modify the fixed buffer table.
> If the uring_cmd is followed by a UBLK_U_IO_UNREGISTER_IO_BUF operation
> that unregisters the fixed buffer, the uring_cmd going async will cause
> the fixed buffer lookup to fail because it happens after the unregister.
> 
> Move the fixed buffer lookup out of io_uring_cmd_import_fixed() and
> instead perform it in io_uring_cmd() before calling ->uring_cmd().
> io_uring_cmd_import_fixed() now only initializes an iov_iter from the
> existing fixed buffer node. This division of responsibilities makes
> sense as the fixed buffer lookup is an io_uring implementation detail
> and independent of the ->uring_cmd() implementation. It also cuts down
> on the need to pass around the io_uring issue_flags.
> 
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> Fixes: 27cb27b6d5ea ("io_uring: add support for kernel registered bvecs")
> ---
>   drivers/nvme/host/ioctl.c    | 10 ++++------
>   include/linux/io_uring/cmd.h |  6 ++----
>   io_uring/rsrc.c              |  6 ++++++
>   io_uring/rsrc.h              |  2 ++
>   io_uring/uring_cmd.c         | 10 +++++++---
>   5 files changed, 21 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/nvme/host/ioctl.c b/drivers/nvme/host/ioctl.c
> index fe9fb80c6a14..3fad74563b9e 100644
> --- a/drivers/nvme/host/ioctl.c
> +++ b/drivers/nvme/host/ioctl.c
> @@ -112,12 +112,11 @@ static struct request *nvme_alloc_user_request(struct request_queue *q,
-- 
Pavel Begunkov