linux-kernel - Re: [PATCH v2 3/3] nvme/ioctl: move fixed buffer lookup to nvme_uring_cmd

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Message-ID: <CADUfDZoMhCyM1FQmhftEeC7G2b5Wj338DE=H6XxrUjp=eOoJ4g@mail.gmail.com>
Date: Thu, 27 Mar 2025 19:47:32 -0700
From: Caleb Sander Mateos <csander@...estorage.com>
To: Christoph Hellwig <hch@....de>
Cc: Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>, 
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] nvme/ioctl: move fixed buffer lookup to nvme_uring_cmd_io()

On Thu, Mar 27, 2025 at 3:44 AM Christoph Hellwig <hch@....de> wrote:
>
> On Fri, Mar 21, 2025 at 02:57:22PM -0600, Caleb Sander Mateos wrote:
> > nvme_map_user_request() is called from both nvme_submit_user_cmd() and
> > nvme_uring_cmd_io(). But the ioucmd branch is only applicable to
> > nvme_uring_cmd_io(). Move it to nvme_uring_cmd_io() and just pass the
> > resulting iov_iter to nvme_map_user_request().
>
> From a quick looks this valid, but actually adds slightly more code.
> Is there some long term plan you have in this area that this helps
> with?  Otherwise it feels like churn.

In v3 [1], the call to io_uring_cmd_import_fixed() is moved before
nvme_alloc_user_request(), which ensures the fixed buffer lookup
happens before the request may go async. That way, the request stills
succeed if the fixed buffer is unregistered before the request can be
reissued in a blocking fashion on the io_uring work queue. There's
also no longer any cleanup required if the lookup fails, so it's a
smaller net increase in lines of code.
Philosophically, it feels like checking the io_uring command flags and
looking up the fixed buffer doesn't really belong in the shared
nvme_map_user_request() helper.
I don't have any particular future plans here. I'd be happy to move
the logic out to a separate helper if you feel nvme_uring_cmd_io() is
getting too big.

Best,
Caleb

[1]: https://lore.kernel.org/lkml/20250324200540.910962-4-csander@purestorage.com/T/#u